Segment and Non-Tariffed Business Reporting (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) subsidiary reportable_segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Subsidiaries | subsidiary									3
Number of Reportable Segments | reportable_segment									2
Operating revenue	$ 93,529	$ 124,578	$ 102,073	$ 69,045	$ 79,306	$ 112,344	$ 86,944	$ 61,112	$ 389,225	$ 339,706	$ 305,082
Utilities Operating Expense									291,146	246,590	225,122
Operating Income (Loss)	18,214	$ 38,245	$ 30,787	$ 10,833	16,389	$ 36,346	$ 29,800	$ 10,581	98,079	93,116	79,960
Net income before noncontrolling interest									61,100	52,839	37,882
Depreciation and amortization									48,292	44,625	40,740
Provision for income taxes									35,393	33,542	23,272
Assets	1,458,001				1,443,376				1,458,001	1,443,376
SJW Corp. [Member]
Segment Reporting Information [Line Items]
Operating revenue									389,225	339,706	305,082
Utilities Operating Expense									291,146	246,590	225,122
Operating Income (Loss)									98,079	93,116	79,960
Net income before noncontrolling interest									61,100	52,839	37,882
Depreciation and amortization									48,292	44,625	40,740
Interest Expense									22,929	21,838	22,186
Provision for income taxes									35,393	33,542	23,272
Assets	1,458,001				1,443,376				1,458,001	1,443,376	1,337,325
Regulated [Member] | Water Utility Services [Member]
Segment Reporting Information [Line Items]
Operating revenue									376,104	326,547	291,949
Utilities Operating Expense									279,833	236,459	214,538
Operating Income (Loss)									96,271	90,088	77,411
Net income before noncontrolling interest									47,736	45,594	38,402
Depreciation and amortization									46,500	42,709	38,742
Interest Expense									20,670	18,667	18,935
Provision for income taxes									30,127	27,902	23,802
Assets	1,406,221				1,368,886				1,406,221	1,368,886	1,250,552
Regulated [Member] | SJW Corp. [Member]
Segment Reporting Information [Line Items]
Operating revenue									376,104	326,547	291,949
Utilities Operating Expense									279,833	236,459	214,538
Operating Income (Loss)									96,271	90,088	77,411
Net income before noncontrolling interest									47,736	45,594	38,402
Depreciation and amortization									46,500	42,709	38,742
Interest Expense									20,670	18,667	18,935
Provision for income taxes									30,127	27,902	23,802
Assets	1,406,221				1,368,886				1,406,221	1,368,886	1,250,552
Nonregulated Operation [Member] | Water Utility Services [Member]
Segment Reporting Information [Line Items]
Operating revenue									7,419	6,442	6,145
Utilities Operating Expense									4,855	4,257	4,149
Operating Income (Loss)									2,564	2,185	1,996
Net income before noncontrolling interest									1,137	954	882
Depreciation and amortization									572	480	434
Interest Expense									0	0	0
Provision for income taxes									993	750	730
Assets	4,471				17,794				4,471	17,794	18,106
Nonregulated Operation [Member] | Real Estate Services [Member]
Segment Reporting Information [Line Items]
Operating revenue									5,702	6,717	6,988
Utilities Operating Expense									3,688	4,074	4,079
Operating Income (Loss)									2,014	2,643	2,909
Net income before noncontrolling interest									8,089	7,406	953
Depreciation and amortization									1,220	1,436	1,564
Interest Expense									60	912	1,019
Provision for income taxes									644	4,235	513
Assets	47,668				54,818				47,668	54,818	65,571
Nonregulated Operation [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenue									0	0	0	[1]
Utilities Operating Expense									2,770	1,800	2,356	[1]
Operating Income (Loss)									(2,770)	(1,800)	(2,356)	[1]
Net income before noncontrolling interest									4,138	(1,115)	(2,355)	[1]
Depreciation and amortization									0	0	0	[1]
Interest Expense									2,199	2,259	2,232	[1]
Provision for income taxes									3,629	655	(1,773)	[1]
Assets	(359)				1,878				(359)	1,878	3,096	[1]
Nonregulated Operation [Member] | SJW Corp. [Member]
Segment Reporting Information [Line Items]
Operating revenue									13,121	13,159	13,133
Utilities Operating Expense									11,313	10,131	10,584
Operating Income (Loss)									1,808	3,028	2,549
Net income before noncontrolling interest									13,364	7,245	(520)
Depreciation and amortization									1,792	1,916	1,998
Interest Expense									2,259	3,171	3,251
Provision for income taxes									5,266	5,640	(530)
Assets	$ 51,780				$ 74,490				$ 51,780	$ 74,490	$ 86,773

[1] The “All Other” category includes the accounts of SJW Group on a stand-alone basis.