General (Policies)	6 Months Ended
General (Policies)	Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Accounting, Policy	The unaudited interim financial information has been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and in accordance with the instructions for Form 10-Q and Rule 10-01 of Regulation S-X promulgated by the Securities and Exchange Commission (the “SEC”). The Notes to Consolidated Financial Statements in SJW Group’s 2016 Annual Report on Form 10-K should be read with the accompanying unaudited condensed consolidated financial statements.
New Accounting Pronouncements, Policy	In March 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2016-09, “Improvements to Employee Share-Based Payment Accounting.” This ASU affects entities that issue share-based payment awards to their employees. ASU 2016-09 identifies areas for simplification involving several aspects of accounting for share-based payment transactions, including income tax consequences, classification of awards as either equity or liabilities, classifications on the statement of cash flows and forfeiture rate calculations. SJW Group adopted this standard as of the first quarter of 2017. ASU 2016-09 requires recognition of excess tax benefits and deficiencies in the income statement, which resulted in the recognition of $500 in income tax benefit for the three months ended March 31, 2017. Prior to adoption, these amounts were recognized as additional paid-in capital. SJW Group did not have any unrecognized excess tax benefits to reclassify upon adoption of this standard. The ASU also requires excess tax benefits and deficiencies to be prospectively excluded from assumed future proceeds in the calculation of diluted shares when calculating diluted earnings per shares using the treasury stock method. The effect of this change on diluted earnings per share was immaterial. In addition, excess income tax benefits from share-based compensation are now classified as cash flows from operating activities on the consolidated statements of cash flows, prospectively. Further, ASU 2016-09 requires, on a retrospective basis, that employee taxes paid for withheld shares be classified as cash flows from financing activities rather than cash flows from operating activities. As such, the consolidated statements of cash flows for SJW Group for the periods presented have been reclassified to reflect this change. This change resulted in an increase to cash flows from operating activities and a decrease to cash flows from financing activities of $815 and $486 for the three months ended March 31, 2017 and 2016, respectively. SJW Group has elected to account for actual forfeitures as they occur upon adoption of the new guidance. Management determined that the cumulative effect adjustment required under the new guidance was immaterial and therefore SJW Group did not record an adjustment.
Earnings Per Share, Policy	Basic earnings per share is calculated using income available to common stockholders, divided by the weighted average number of shares outstanding during the period. Diluted earnings per share is calculated using income available to common stockholders divided by the weighted average number of shares of common stock including both shares outstanding and shares potentially issuable in connection with deferred restricted common stock awards under SJW Group’s Long-Term Incentive Plan (as amended, the “Incentive Plan”) and shares potentially issuable under the 2014 Employee Stock Purchase Plan (“ESPP”).