Segment and Non-Tariffed Business Reporting (Details) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) subsidiary reportable_segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of subsidiaries | subsidiary				4
Number of reportable business segments | reportable_segment				2
Operating revenue		$ 113,997	$ 124,853	$ 294,644	$ 298,981
Operating expense		96,913	99,025	243,181	243,023
Operating income (loss)		17,084	25,828	51,463	55,958
Net income (loss)		9,478	15,788	28,889	29,944
Depreciation and amortization		15,122	13,682	45,368	40,921
Senior note and other interest expense		6,588	6,077	19,093	18,213
Income tax expense (benefit) in net income		2,565	4,103	8,802	7,591
Assets		2,022,382	1,536,868	2,022,382	1,536,868	$ 1,956,389
Water Utility Services | Regulated
Segment Reporting Information [Line Items]
Operating revenue		110,218	121,009	285,138	289,160
Operating expense		89,924	87,230	226,073	219,573
Operating income (loss)		20,294	33,779	59,065	69,587
Net income (loss)		10,964	22,333	30,726	42,150
Depreciation and amortization		14,712	13,298	44,159	39,771
Senior note and other interest expense		6,044	5,534	17,434	16,582
Income tax expense (benefit) in net income		2,912	5,910	9,142	11,052
Assets		1,556,049	1,480,726	1,556,049	1,480,726
Water Utility Services | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		2,426	2,403	5,418	5,718
Operating expense		1,636	1,623	3,833	3,789
Operating income (loss)		790	780	1,585	1,929
Net income (loss)		555	561	1,127	1,389
Depreciation and amortization		111	85	313	253
Senior note and other interest expense		0	0	0	0
Income tax expense (benefit) in net income		216	218	438	540
Assets		5,899	4,577	5,899	4,577
Real Estate Services | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		1,353	1,441	4,088	4,103
Operating expense		935	934	2,834	2,674
Operating income (loss)		418	507	1,254	1,429
Net income (loss)		304	359	1,284	1,011
Depreciation and amortization		299	299	896	897
Senior note and other interest expense		0	0	0	0
Income tax expense (benefit) in net income		95	113	445	299
Assets		46,160	47,469	46,160	47,469
All Other | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue	[1]	0	0	0	0
Operating expense	[1]	4,418	9,238	10,441	16,987
Operating income (loss)	[1]	(4,418)	(9,238)	(10,441)	(16,987)
Net income (loss)	[1]	(2,345)	(7,465)	(4,248)	(14,606)
Depreciation and amortization	[1]	0	0	0	0
Senior note and other interest expense	[1]	544	543	1,659	1,631
Income tax expense (benefit) in net income	[1]	(658)	(2,138)	(1,223)	(4,300)
Assets	[1]	414,274	4,096	414,274	4,096
SJW Group | Regulated
Segment Reporting Information [Line Items]
Operating revenue		110,218	121,009	285,138	289,160
Operating expense		89,924	87,230	226,073	219,573
Operating income (loss)		20,294	33,779	59,065	69,587
Net income (loss)		10,964	22,333	30,726	42,150
Depreciation and amortization		14,712	13,298	44,159	39,771
Senior note and other interest expense		6,044	5,534	17,434	16,582
Income tax expense (benefit) in net income		2,912	5,910	9,142	11,052
Assets		1,556,049	1,480,726	1,556,049	1,480,726
SJW Group | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		3,779	3,844	9,506	9,821
Operating expense		6,989	11,795	17,108	23,450
Operating income (loss)		(3,210)	(7,951)	(7,602)	(13,629)
Net income (loss)		(1,486)	(6,545)	(1,837)	(12,206)
Depreciation and amortization		410	384	1,209	1,150
Senior note and other interest expense		544	543	1,659	1,631
Income tax expense (benefit) in net income		(347)	(1,807)	(340)	(3,461)
Assets		$ 466,333	$ 56,142	$ 466,333	$ 56,142

[1] *    The “All Other” category includes the accounts of SJW Group and Hydro Sub, Inc. on a stand-alone basis