Real Estate Investments, Net [Abstract] Schedule of Real Estate Investments Schedule of Real Estate Investments [Table Text Block] Tabular disclosure of real estate investments. Regulatory Assets and Liabilities Disclosure [Abstract] REGULARTORY ASSETS ANAD LIABILITIES Schedule of Regulatory Assets and Liabilities [Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Ownership [Axis] Ownership [Axis] Ownership [Domain] Ownership [Domain] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Business Acquisition [Line Items] Business Acquisition [Line Items] Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] GENERAL Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Regulated Operations [Abstract] Public Utilities General Disclosures Public Utilities General Disclosures [Table Text Block] Segment Reporting [Abstract] SEGMENT AND NONTARIFFED BUSINESS REPORTING Segment Reporting Disclosure [Text Block] Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Statement of Financial Position [Abstract] Common stock, par value (usd per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized (shares) Common Stock, Shares Authorized Common stock, shares issued (shares) Common Stock, Shares, Issued Common stock, shares outstanding (shares) Common Stock, Shares, Outstanding Schedule of Regulatory Assets [Table] Schedule of Regulatory Assets [Table] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Deferred Income Tax Charge [Member] Deferred Income Tax Charge [Member] Postretirement pensions and other medical benefits Postretirement Pensions and Other Medical Benefits [Member] Postretirement Pensions and Other Medical Benefits [Member] Business combinations debt premium, net [Member] Business combinations debt premium, net [Member] Business combinations debt premium, net [Member] Balancing and memorandum accounts, net Balancing and Memorandum Accounts [Member] Balancing and Memorandum Accounts [Member] WRA [Member] WRA [Member] WRA [Member] Other, net Other Regulatory Assets [Member] Other Regulatory Assets [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Regulatory Assets [Line Items] Regulatory Assets [Line Items] Regulatory assets: Regulatory Assets Net Regulatory Assets Net Regulatory Assets Less: current regulatory asset, net Regulatory Assets, Current Total regulatory assets, net, less current portion Regulatory Assets, Noncurrent Income Statement [Abstract] REVENUE Revenues OPERATING EXPENSE: Costs and Expenses [Abstract] Production Expenses: Water Production Costs [Abstract] Water Production Costs [Abstract] Purchased water Cost of Purchased Water Power Utilities Operating Expense, Purchased Power Groundwater extraction charges Groundwater Extraction Charges Groundwater Extraction Charges Other production expenses Utilities Operating Expense, Other Total production expenses Water Production Costs Administrative and general General and Administrative Expense Maintenance Utilities Operating Expense, Maintenance Property taxes and other non-income taxes Utilities Operating Expense, Taxes Depreciation and amortization Utilities Operating Expense, Depreciation and Amortization Merger related expenses Business Combination, Acquisition Related Costs Total operating expense Utilities Operating Expense OPERATING INCOME Operating Income (Loss) OTHER (EXPENSE) INCOME: Other Nonoperating Income (Expense) [Abstract] Interest on long-term debt and other interest expense Interest Expense, Debt Pension non-service cost Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component Interest income on money market fund Interest Income, Money Market Deposits Gain on sale of real estate investments Gain (Loss) on Sale of Properties Other, net Other Nonoperating Income (Expense) Income before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Provision for income taxes Income Tax Expense (Benefit) NET INCOME BEFORE NONCONTROLLING INTEREST Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Less net income attributable to the noncontrolling interest Net Income (Loss) Attributable to Noncontrolling Interest SJW GROUP NET INCOME Net Income (Loss) Attributable to Parent Other comprehensive income (loss), net Other Comprehensive Income (Loss), Net of Tax SJW GROUP COMPREHENSIVE INCOME Comprehensive Income (Loss), Net of Tax, Attributable to Parent SJW GROUP EARNINGS PER SHARE Earnings Per Share [Abstract] Basic (usd per share) Earnings Per Share, Basic Diluted (usd per share) Earnings Per Share, Diluted DIVIDENDS PER SHARE Common Stock, Dividends, Per Share, Cash Paid WEIGHTED AVERAGE SHARES OUTSTANDING Weighted Average Number of Shares Outstanding Basic and Diluted [Abstract] Weighted Average Number of Shares Outstanding Basic and Diluted [Abstract] Basic (shares) Weighted Average Number of Shares Outstanding, Basic Diluted (shares) Weighted Average Number of Shares Outstanding, Diluted REAL ESTATE INVESTMENTS SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation Disclosure [Text Block] Share-based Payment Arrangement [Abstract] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan Share-based Payment Arrangement, Cost by Plan [Table Text Block] Long-Term Liabilities and Bank Borrowings [Abstract] Long-Term Liabilities and Bank Borrowings [Abstract] LONG-TERM LIABILITIES AND BANK BORROWINGS Long-term Debt [Text Block] BALANCING AND MEMORANDUM ACCOUNTS Public Utilities Disclosure [Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Fed Funds Effective Rate Fed Funds Effective Rate Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Connecticut Water Service, Inc. [Member] Connecticut Water Service, Inc. [Member] Connecticut Water Service, Inc. [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Revolving Credit Facility - Temporary [Member] Revolving Credit Facility - Temporary [Member] Revolving Credit Facility - Temporary [Member] Lender Name [Axis] Lender Name [Axis] Line of Credit Facility, Lender [Domain] Line of Credit Facility, Lender [Domain] Chase Bank, N.A. [Member] Chase Bank, N.A. [Member] Chase Bank, N.A. [Member] CoBank [Member] CoBank [Member] CoBank [Member] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] SJW Corp. and SJW Land Company [Member] SJW Corp. and SJW Land Company [Member] SJW Corp. and SJW Land Company [Member] San Jose Water Company [Member] San Jose Water Company [Member] San Jose Water Company [Member] Connecticut Water Company [Member] Connecticut Water Company [Member] Connecticut Water Company [Member] Maine Water Company [Member] Maine Water Company [Member] Maine Water Company [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Line of Credit [Member] Line of Credit [Member] Senior Notes [Member] Senior Notes [Member] Public Utility, Bonds [Member] Public Utility, Bonds [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] State Revolving Fund Loans Series S [Member] State Revolving Fund Loans Series S [Member] State Revolving Fund Loans Series S [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Debt Instrument, Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Line of Credit Facility, Increase in Maximum Borrowing Capacity Line of Credit Facility, Increase in Maximum Borrowing Capacity Line of Credit Facility, Increase in Maximum Borrowing Capacity Principal Debt Sold Principal Debt Sold Principal Debt Sold Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Increase (Decrease) in Restricted Cash Increase (Decrease) in Restricted Cash Ratio of Indebtedness to Capital Ratio of Indebtedness to Capital Ratio of Indebtedness to Total Capital Long-term Debt Long-term Debt Statement of Cash Flows [Abstract] OPERATING ACTIVITIES: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Deferred income taxes Deferred Income Tax Expense (Benefit) Stock-based compensation Share-based Payment Arrangement, Noncash Expense Gain (Loss) on Sale of Properties Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Accounts receivable and accrued unbilled utility revenue Increase (Decrease) in Receivables Accounts payable and other current liabilities Increase (Decrease) in Accounts Payable Accrued groundwater extraction charges, purchased water and power Increase (Decrease) in Accrued Groundwater Extraction Charges, Purchased Water and Power Increase (Decrease) in Accrued Groundwater Extraction Charges, Purchased Water and Power Tax payable and receivable, and other accrued taxes Increase (Decrease) in Accrued Taxes Payable Postretirement benefits Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits Regulatory assets and liabilities related to balancing and memorandum accounts Increase (Decrease) to Regulatory Asset Related to Balancing and Memorandum Accounts Increase (Decrease) to Regulatory Asset Related to Balancing and Memorandum Accounts Up-front service concession payment Increase (Decrease) In Up-Front Service Concession Payment Increase (Decrease) In Up-Front Service Concession Payment Other noncurrent assets and noncurrent liabilities increase (Decrease) in Other Noncurrent Assets and Liabilities The increase (decrease) during the reporting period in other noncurrent operating assets and liabilities not seperately disclosed in the statement of cash flows. Other changes, net Increase (Decrease) in Other Operating Assets and Liabilities, Net NET CASH PROVIDED BY OPERATING ACTIVITIES Net Cash Provided by (Used in) Operating Activities INVESTING ACTIVITIES: Net Cash Provided by (Used in) Investing Activities [Abstract] Company-funded Payments to Acquire Water Systems Contributions in aid of construction Payments to Acquire Water Systems using Contributions In Aid of Construction Payments to Acquire Water Systems using Contributions In Aid of Construction Additions to real estate investments Payments to Acquire Real Estate Payments to retire utility plant, net of salvage Payments for (Proceeds from) Removal Costs Proceeds from Sale of Property Held-for-sale Proceeds from Sale of Property Held-for-sale NET CASH USED IN INVESTING ACTIVITIES Net Cash Provided by (Used in) Investing Activities FINANCING ACTIVITIES: Net Cash Provided by (Used in) Financing Activities [Abstract] Borrowings on line of credit Proceeds from Lines of Credit Repayments on line of credit Repayments of Lines of Credit Long-term borrowings Proceeds from Issuance of Long-term Debt Repayments of long-term borrowings Repayments of Long-term Debt Debt issuance and broker fee costs Payments of Debt Issuance Costs Dividends paid Payments of Ordinary Dividends, Common Stock Receipts of advances and contributions in aid of construction Receipts Of Advances And Contributions In Aid Of Construction Receipts Of Advances And Contributions In Aid Of Construction Refunds of advances for construction Repayments of Advances for Construction Other changes, net Proceeds from (Payments for) Other Financing Activities NET CASH PROVIDED BY FINANCING ACTIVITIES Net Cash Provided by (Used in) Financing Activities NET CHANGE IN CASH AND CASH EQUIVALENTS Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents CASH AND CASH EQUIVALENTS, END OF PERIOD Cash paid during the period for: Supplemental Cash Flow Information [Abstract] Interest Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes Income Taxes Paid, Net Supplemental disclosure of non-cash activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Change in accrued payables for construction costs capitalized Noncash Change in Accrued Payables Noncash Change in Accrued Payables Utility property installed by developers Utility Property Installed by Developer Utility Property Installed by Developer Accounting Policies [Abstract] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Restricted Stock and Stock Options Restricted Stock and Stock Options [Member] Restricted Stock and Stock Options [Member] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Anti-dilutive restricted common stock units excluded from computation of earnings per share (shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Retirement Benefits [Abstract] Schedule of Net Benefit Costs Schedule of Net Benefit Costs [Table Text Block] Schedule of Allocation of Plan Assets Schedule of Allocation of Plan Assets [Table Text Block] Schedule of Regulatory Assets Schedule of Regulatory Assets [Table Text Block] Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Cost of capital memorandum account Cost of capital [Member] Cost of capital [Member] Avon Water Company [Member] Avon Water Company [Member] Avon Water Company [Member] SJWTX, Inc. [Member] SJWTX, Inc. [Member] SJWTX, Inc. [Member] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] CPED Calculated Refund 2014-2016 CPED Calculated Refund 2014-2016 [Member] CPED Calculated Refund 2014-2016 [Member] CPED Calculated Refund 1987-2013 CPED Calculated Refund 1987-2013 [Member] CPED Calculated Refund 1987-2013 [Member] OII No. 18-09-003 Settlement Agreement OII No. 18-09-003 Settlement Agreement [Member] OII No. 18-09-003 Settlement Agreement [Member] Advice Letter No. 545 [Member] Advice Letter No. 545 [Member] Advice Letter No. 545 Advice Letter No. 534 [Member] Advice Letter No. 534 [Member] Advice Letter No. 534 [Member] Advice Letter No. 548 [Member] Advice Letter No. 548 [Member] Advice Letter No. 548 Advice Letter No. 537 Advice Letter No. 537 [Member] Advice Letter No. 537 [Member] Advice Letter No. 541 [Member] Advice Letter No. 541 [Member] Advice Letter No. 541 [Member] Application No. 19-12-002 [Member] Application No. 19-12-002 [Member] Application No. 19-12-002 [Member] Cost Of Capital Filings [Member] Cost Of Capital Filings [Member] Cost Of Capital Filings [Member] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Public Utilities, General Disclosures [Line Items] Public Utilities, General Disclosures [Line Items] Regulatory Asset Increase (Decrease) Balancing and Memorandum Revenue Account, Increase (Decrease) Balancing and Memorandum Revenue Account, Increase (Decrease) Memorandum Account, Requested Recovery Memorandum Account, Requested Recovery (Refund) Memorandum Account, Requested Recovery (Refund) CPED Calculated Service Charge Refund CPED Calculated Service Charge Refund CPED Proposed Service Charge Refund CPED Calculated Double-Billing Refund CPED Calculated Double-Billing Refund CPED Calculated Double-Billing Refund CPED Proposed Penalty, Per Offense, Per Day CPED Proposed Penalty, Per Offense, Per Day CPED Proposed Penalty, Per Offense, Per Day Customer Credit Refund Customer Credit Refund Customer Credit Refund Utility Plant Investment Commitment Utility Plant Investment Commitment Utility Plant Investment Commitment Memorandum Account, Requested Refund, Monthly Amount Memorandum Account, Requested Recovery (Refund), Monthly Amount Memorandum Account, Requested Recovery (Refund), Monthly Amount Public Utilities, Requested Rate Increase (Decrease), Amount Public Utilities, Requested Rate Increase (Decrease), Amount Public Utilities, Requested Rate Increase (Decrease), Percentage Public Utilities, Requested Rate Increase (Decrease), Percentage Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 1 Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 1 Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 1 Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 2 Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 2 Public Utilities, Approved Rate Increase (Decrease), Percentage, Step Increase 2 Public Utilities, Approved Rate Increase (Decrease), Percentage Public Utilities, Approved Rate Increase (Decrease), Percentage Regulatory Rate Filings, Requested Rate Increase, Year One Regulatory Rate Filings, Requested Rate Increase, Year One Regulatory Rate Filings, Requested Rate Increase, Year One Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year One Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year One Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year One Regulatory Rate Filings, Requested Rate Increase, Year Two Regulatory Rate Filings, Requested Rate Increase, Year Two Regulatory Rate Filings, Requested Rate Increase, Year Two Regulatory Rate Filings, Proposed Rate Increase, Percent of Authorized Revenue, Year Two Regulatory Rate Filings, Proposed Rate Increase, Percent of Authorized Revenue, Year Two Regulatory Rate Filings, Proposed Rate Increase, Percent of Authorized Revenue, Year Two Regulatory Rate Filings, Requested Rate Increase, Year Three Regulatory Rate Filings, Requested Rate Increase, Year Three Regulatory Rate Filings, Requested Rate Increase, Year Three Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Three Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Three Regulatory Rate Filings, Requested Rate Increase as Percentage of Total Revenue at Time of Request, Year Three Regulatory Rate Filings, Requested Rate Increase, Year Four Regulatory Rate Filings, Requested Rate Increase, Year Four Regulatory Rate Filings, Requested Rate Increase, Year Four Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four Regulatory Rate Filings, Requested Rate Increase, Percent of Authorized Revenue, Year Four Regulatory Rate Filings, Requested Deferment Period Regulatory Rate Filings, Requested Deferment Period Regulatory Rate Filings, Requested Deferment Period Authorized WICA Surcharge, Percentage Authorized WICA Surcharge, Percentage Authorized WICA Surcharge, Percentage Requested WICA Surcharge Increase (Decrease), Percentage Requested WICA Surcharge Increase (Decrease), Percentage Requested WICA Surcharge Increase (Decrease), Percentage Requested WICA Surcharge, Percentage Requested WICA Surcharge, Percentage Requested WICA Surcharge, Percentage Authorized Regulatory Surcharge, Per Thousand Gallons Authorized Regulatory Surcharge, Per Thousand Gallons Authorized Regulatory Surcharge, Per Thousand Gallons Authorized Regulatory Base Charge, Per Residential Account Authorized Regulatory Base Charge, Per Residential Account Authorized Regulatory Base Charge, Per Residential Account Requested WISC Surcharge Increase (Decrease), Percentage Requested WISC Surcharge Increase (Decrease), Percentage Requested WISC Surcharge Increase (Decrease), Percentage Statement of Stockholders' Equity [Abstract] Statement [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock Common Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income AOCI Attributable to Parent [Member] Noncontrolling Interest [Member] Noncontrolling Interest [Member] Statement [Line Items] Statement [Line Items] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning balance Stockholders' Equity Attributable to Parent Beginning balance (in shares) Net income Cumulative effect of change in accounting principle, net of tax effect of $33 Cumulative Effect of New Accounting Principle in Period of Adoption Distribution to noncontrolling interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Unrealized Gain (Loss) on Investments Unrealized Gain (Loss) on Investments Share-based compensation APIC, Share-based Payment Arrangement, Increase for Cost Recognition Issuance of restricted and deferred stock units Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Issuance of restricted and deferred stock units (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Employee stock purchase plan Stock Issued During Period, Value, Employee Stock Purchase Plan Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Employee stock purchase plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Dividends paid Dividends Ending balance Ending balance (in shares) DEFINED BENEFIT PLAN Pension and Other Postretirement Benefits Disclosure [Text Block] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Domain] Defined Benefit Plan, Plan Assets, Category [Domain] Cash and cash equivalents Cash and Cash Equivalents [Member] Actively Managed Actively Managed Investments [Member] Actively Managed Investments [Member] Fixed Income Fixed Income Funds [Member] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Equity securities (a) Equity Securities [Member] Fixed income (b) Fixed Income Securities [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy and NAV [Domain] Quoted Prices in Active Markets for Identical Assets Fair Value, Inputs, Level 1 [Member] Significant Observable Inputs Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs Fair Value, Inputs, Level 3 [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Components of Net Periodic Benefit Cost [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Other cost Defined Benefit Plan, Other Cost (Credit) Expected return on assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Plan Assets [Abstract] Plan Assets [Abstract] Plan Assets [Abstract] Fair value of plan assets Defined Benefit Plan, Plan Assets, Amount Employer Contributions [Abstract] Payment for Pension and Other Postretirement Benefits [Abstract] Estimated employer contributions for the current fiscal year Defined Benefit Plan, Estimated Total Employer Contributions for the Current Fiscal Year The employer's best estimate of total contributions expected to be paid to the plan for the current fiscal period. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. Defined Contribution Plan, Employer Discretionary Contribution Amount Defined Contribution Plan, Employer Discretionary Contribution Amount Geographic Distribution [Axis] Geographic Distribution [Axis] Geographic Distribution [Domain] Geographic Distribution [Domain] Southbury, Connecticut [Member] Southbury, Connecticut [Member] Southbury, Connecticut [Member] Series 2019A/B/C [Member] Series 2019A/B/C [Member] Series 2019A/B/C [Member] Sale of Stock, Type of Offering [Axis] Sale of Stock, Type of Offering [Axis] Sales of Stock,Type of Offering [Axis] Type of Offering [Domain] Type of Offering [Domain] [Domain] for Type of Offering [Axis] Underwritten Public Offering [Member] Underwritten Public Offering [Member] Underwritten Public Offering [Member] Parent Company [Member] Parent Company [Member] CTWS [Member] CTWS [Member] CTWS [Member] Public Utilities, Service Connections Public Utilities, Service Connections Public Utilities, Service Connections Public Utilities, Number of People Served Public Utilities, Number of People Served Public Utilities, Number of People Served Public Utilities, Number of Communities Served Public Utilities, Number of Communities Served Public Utilities, Number of Communities Served Business Combination, Consideration Transferred, Acquiree's Common Shares Converted In Cash Per Share Business Combination, Consideration Transferred, Acquiree's Common Shares Converted In Cash Per Share Business Combination, Consideration Transferred, Acquiree's Common Shares Converted In Cash Per Share Payments to Acquire Businesses, Gross Payments to Acquire Businesses, Gross Cash Acquired from Acquisition Cash Acquired from Acquisition Adjustments to Additional Paid in Capital, Share-based Compensation, Equity Award Assumed in Acquisition Adjustments to Additional Paid in Capital, Share-based Compensation, Equity Award Assumed in Acquisition Adjustments to Additional Paid in Capital, Share-based Compensation, Equity Award Assumed in Acquisition Stock Issued During Period, Shares, New Issues Stock Issued During Period, Shares, New Issues Proceeds from Issuance of Common Stock, Gross Proceeds from Issuance of Common Stock, Gross Proceeds from Issuance of Common Stock, Gross Assets Acquired Assets [Abstract] Utility plant, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Utility Plan, Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Utility Plan, Net Nonutility plant Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Nonutility Plant, Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Nonutility Plant, Net Current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other Investments Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Assets Regulatory assets and deferred charges, less current portion Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Regulatory Asses and Deferred Charges, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Regulatory Asses and Deferred Charges, net Other intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Other assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets Goodwill Goodwill Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Post-retirement Benefit Plans Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Post-retirement Benefit Plans Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Post-retirement Benefit Plans Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, CIAC and Advances Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, CIAC and Advances Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, CIAC and Advances Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net BUSINESS COMBINATION Business Combination Disclosure [Text Block] Public Utility, Property, Plant and Equipment [Table] Public Utility, Property, Plant and Equipment [Table] General and Administrative Expense General and Administrative Expense [Member] Public Utility, Property, Plant and Equipment [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Depreciation Depreciation Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Entity Listing, Description Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Document Transition Report Document Transition Report Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Incentive Plan Incentive Plan [Member] Incentive Plan [Member] CTWS Plan [Member] CTWS Plan [Member] CTWS Plan [Member] Employee Stock Purchase Plan (ESPP) Employee Stock Purchase Plan [Member] Employee Stock Purchase Plan [Member] Award Type [Axis] Award Type [Axis] Award Type [Domain] Award Type [Domain] Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Performance Shares Performance Shares [Member] Market-based RSU [Member] Market-based RSU [Member] Market-based RSU [Member] Restricted stock and deferred restricted stock Restricted Stock and Deferred Restricted Stock [Member] Restricted Stock and Deferred Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Shares issuable upon exercise of Incentive Plan awards (shares) Share-based Compensation Arrangement by Share-based Payment Award, Shares Issuable upon Exercise Number of shares reserved for issuance under equity instrument agreements awarded that validly exist and are outstanding, including vested instruments. Remaining shares available for issuance under the Incentive Plan (shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Compensation costs charged to income: Share-based Payment Arrangement, Expense ESPP proceeds Proceeds, Issuance of Shares, Share-based Payment Arrangement, Including Option Exercised Number of equity instruments granted (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Grant date fair value of equity instruments granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Target vesting percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Unrecognized compensation costs Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Recognition period for unrecognized compensation cost Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Purchase price of common stock under ESPP (percent) Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent Maximum percentage of base compensation employees can designate for stock purchases under ESPP (percent) Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate Number of shares authorized for issuance under the plan (shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Plan expense Stock or Unit Option Plan Expense REGULATORY RATE FILINGS Regulatory Rate Filings [Text Block] Regulatory Rate Filings [Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Water Utility Services Reportable Segment One [Member] Reportable Segment One [Member] Real Estate Services Reportable Segment Two [Member] Reportable Segment Two [Member] All Other Other Segments [Member] SJW Group Corporate Segment [Member] Regulation Status [Axis] Regulation Status [Axis] Regulated and Unregulated Operation [Domain] Regulated and Unregulated Operation [Domain] Regulated Regulated Operation [Member] Non-tariffed Unregulated Operation [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Number of subsidiaries Number of Subsidiaries Number of Subsidiaries Number of reportable business segments Number of Reportable Segments Operating revenue Regulated Operating Revenue Operating expense Operating income (loss) Net income (loss) Senior note and other interest expense Interest Expense Income tax expense (benefit) in net income Assets Assets Fair Value Disclosures [Abstract] FAIR VALUE MEASUREMENT Fair Value Disclosures [Text Block] Revenue from contracts with customers Revenue from Contract with Customer, Excluding Assessed Tax Alternative revenue programs, net Alternative Revenue Programs, Net Alternative Revenue Programs, Net Other balancing and memorandum accounts revenue, net Regulatory Balancing and Memorandum Accounts Revenue, Net Regulatory Balancing and Memorandum Accounts Revenue, Net Rental income Operating Leases, Income Statement, Lease Revenue Total revenues Basis of Accounting, Policy Basis of Accounting, Policy [Policy Text Block] Recently Adopted Accounting Pronouncements, Policy New Accounting Pronouncements, Policy [Policy Text Block] Revenue Recognition, Policy Revenue from Contract with Customer [Policy Text Block] Earnings Per Share, Policy Earnings Per Share, Policy [Policy Text Block] Utility Plant Depreciation, Policy Regulatory Depreciation and Amortization, Policy [Policy Text Block] Commitments and Contingencies Disclosure [Abstract] LEGAL PROCEEDINGS Commitments and Contingencies Disclosure [Text Block] Monterey WRAM [Member] Monterey WRAM [Member] Monterey WRAM 2014-2017 WCMA 2014-2017 WCMA [Member] 2014-2017 WCMA [Member] 2018 WCMA 2018 WCMA [Member] 2018 WCMA [Member] 2019 WCMA 2019 WCMA [Member] 2019 WCMA [Member] 2012 General Rate Case true-up 2012 General Rate Case True-up [Member] 2012 General Rate Case True-up [Member] Tax memorandum account Tax memorandum [Member] Tax memorandum account [Member] All others Water supply costs Water Supply Balancing Accounts [Member] Water Supply Balancing Accounts [Member] Pension Pension Balancing Account [Member] Pension Balancing Account [Member] WCMA Reserve WCMA Reserve WCMA Reserve Memorandum Account, Revenue Increase (Reduction), Gross Memorandum Account, Revenue Increase (Reduction), Gross Memorandum Account, Revenue Increase (Reduction), Gross Balancing and Memorandum Revenue Account [Roll Forward] Balancing and Memorandum Revenue Account [Roll Forward] Balancing and Memorandum Revenue Account [Roll Forward] Beginning Balance Balancing and Memorandum Revenue Account, Net Balancing and Memorandum Revenue Accounting, Net Balancing and Memorandum Revenue Account, Increase (Decrease) Refunds (Collections) Adjustments Balancing and Memorandum Revenue Account, Refunds (Collections) Balancing and Memorandum Revenue Account, Refunds (Collections) Ending Balance Balancing and Memorandum Cost Recovery Account [Roll Forward] Balancing and Memorandum Cost Recovery Account [Roll Forward] Balancing and Memorandum Cost Recovery Account [Roll Forward] Beginning Balance Balancing and Memorandum Cost Recovery Account, Net Balancing and Memorandum Cost Recovery Account, Net Regulatory Asset Increase (Decrease) Balancing and Memorandum Cost Recovery Account, Increase (Decrease) Balancing and Memorandum Cost Recovery Account, Increase (Decrease) Refunds (Collections) Adjustments Balancing and Memorandum Cost Recovery Account, Refunds (Collections) Balancing and Memorandum Cost Recovery Account, Refunds (Collections) Ending Balance Balancing and Memorandum Account [Roll Forward] Balancing and Memorandum Account [Roll Forward] Balancing and Memorandum Account [Roll Forward] Beginning Balance Balancing and Memorandum Account, Net Balancing and Memorandum Account, Net Regulatory Asset Increase (Decrease) Balancing and Memorandum Account, Increase (Decrease) Balancing and Memorandum Account, Increase (Decrease) Refunds (Collections) Balancing and Memorandum Account, Refunds (Collections) Balancing and Memorandum Account, Refunds (Collections) Ending Balance Total balance, net under-collection amount Regulatory Balancing and Memorandum Accounts, Net Under-Collected Regulatory Balancing and Memorandum Accounts,, Net Under-Collected Authorized revenue, threshold percentage Regulatory Balancing and Memorandum Accounts Review, Authorized Revenue, Threshold Percentage Threshold balance of regulatory balancing or memorandum accounts as a percentage of authorized revenue to trigger an account review. ASSETS Utility plant: Public Utilities, Property, Plant and Equipment, Net [Abstract] Land Public Utilities, Property, Plant and Equipment, Land Depreciable plant and equipment Public Utilities, Property, Plant and Equipment, Equipment Construction in progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Intangible assets Finite-Lived Intangible Assets, Gross Utility plant, gross Property, Plant and Equipment, Gross Less accumulated depreciation and amortization Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Utility plant, net Public Utilities, Property, Plant and Equipment, Net Real estate investments Real Estate Investment Property, at Cost Less accumulated depreciation and amortization Real Estate Investment Property, Accumulated Depreciation Total Property, Plant and Equipment, Net CURRENT ASSETS: Assets, Current [Abstract] Cash and cash equivalents: Cash, Cash Equivalents, and Short-term Investments [Abstract] Cash Cash Restricted cash Restricted Cash and Cash Equivalents, Current Accounts receivable: Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Customers, net of allowances for uncollectible accounts Accounts Receivable, after Allowance for Credit Loss, Current Income tax Income Taxes Receivable, Current Other Nontrade Receivables, Current Accrued unbilled utility revenue Receivables, Long-term Contracts or Programs Current regulatory assets, net Other current assets Other Assets, Current Current assets Assets, Current OTHER ASSETS: Other Assets, Noncurrent [Abstract] Net regulatory assets, less current portion Investments Investments Other Other Assets, Noncurrent Other assets Other Assets Assets CAPITALIZATION AND LIABILITIES Liabilities and Equity [Abstract] CAPITALIZATION: Capitalization, Long-term Debt and Equity [Abstract] Stockholders’ equity: Stockholders' Equity, Number of Shares, Par Value and Other Disclosures [Abstract] Common stock, $0.001 par value; authorized 70,000,000 shares; issued and outstanding shares 28,516,705 on June 30, 2020 and 28,456,508 on December 31, 2019 Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax Total stockholders’ equity Long-term debt, less current portion Long-term Debt, Excluding Current Maturities Capitalization, Long-term Debt and Equity Capitalization, Long-term Debt and Equity CURRENT LIABILITIES: Liabilities, Current [Abstract] Line of credit Line of Credit, Current Current portion of long-term debt Long-term Debt, Current Maturities Accrued groundwater extraction charges, purchased water and power Accrued groundwater extraction charges, purchased water and power Accrued groundwater extraction charges, purchased water and power Accounts payable Accounts Payable, Current Accrued interest Interest Payable, Current Accrued payroll Employee-related Liabilities, Current Other current liabilities Other Liabilities, Current Current liabilities Liabilities, Current DEFERRED INCOME TAXES Deferred Income Tax Liabilities, Net ADVANCES FOR CONSTRUCTION Contract with Customer, Liability, Noncurrent CONTRIBUTIONS IN AID OF CONSTRUCTION Contributions in Aid of Construction POSTRETIREMENT BENEFIT PLANS Liability, Defined Benefit Plan, Noncurrent OTHER NONCURRENT LIABILITIES Other Liabilities, Noncurrent COMMITMENTS AND CONTINGENCIES Commitments and Contingencies Capitalization and liabilities Liabilities and Equity Major Streams of Revenue Disaggregation of Revenue [Table Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Money Market Funds [Member] Money Market Funds [Member] Mutual Fund [Member] Mutual Fund [Member] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Supplemental Employee Retirement Plan [Member] Supplemental Employee Retirement Plan [Member] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Life Insurance, Corporate or Bank Owned, Amount Life Insurance, Corporate or Bank Owned, Amount Cash Surrender Value of Life Insurance Cash Surrender Value of Life Insurance Long-term debt, fair value Long-term Debt, Fair Value Cumulative effect of change in accounting principle, taxes Cumulative Effect on Retained Earnings, Tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Schedule of Investments [Table] Schedule of Investments [Table] Schedule of Investments [Line Items] Schedule of Investments [Line Items] Proceeds from Sale of Real Estate Held-for-investment Proceeds from Sale of Real Estate Held-for-investment Land Land Buildings and improvements Investment Building and Building Improvements Subtotal Less accumulated depreciation and amortization Total Estimated useful life SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Life Used for Depreciation EQUITY PLANS Shareholders' Equity and Share-based Payments [Text Block]