Segment and Non-Tariffed Business Reporting (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) subsidiary reportable_segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of subsidiaries | subsidiary				4
Number of reportable business segments | reportable_segment				2
Operating revenue		$ 102,965	$ 99,086	$ 180,647	$ 174,128
Operating expense		80,994	76,287	146,268	143,998
Operating income (loss)		21,971	22,799	34,379	30,130
Net income (loss)		13,538	12,871	19,411	14,156
Depreciation and amortization		15,101	13,656	30,246	27,239
Senior note and other interest expense		6,714	6,084	12,505	12,136
Income tax expense (benefit) in net income		4,192	4,068	6,237	3,488
Assets		2,003,650	1,500,121	2,003,650	1,500,121	$ 1,956,389
Water Utility Services | Regulated
Segment Reporting Information [Line Items]
Operating revenue		99,773	95,798	174,920	168,151
Operating expense		75,564	70,642	136,149	132,343
Operating income (loss)		24,209	25,156	38,771	35,808
Net income (loss)		13,662	15,022	19,762	19,817
Depreciation and amortization		14,698	13,272	29,447	26,473
Senior note and other interest expense		6,170	5,540	11,390	11,048
Income tax expense (benefit) in net income		4,154	4,650	6,230	5,142
Assets		1,537,404	1,449,714	1,537,404	1,449,714
Water Utility Services | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		1,823	1,967	2,992	3,315
Operating expense		1,356	1,278	2,197	2,166
Operating income (loss)		467	689	795	1,149
Net income (loss)		336	497	572	828
Depreciation and amortization		105	85	202	168
Senior note and other interest expense		0	0	0	0
Income tax expense (benefit) in net income		130	193	222	322
Assets		6,149	3,768	6,149	3,768
Real Estate Services | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		1,369	1,321	2,735	2,662
Operating expense		1,008	891	1,899	1,740
Operating income (loss)		361	430	836	922
Net income (loss)		662	296	980	652
Depreciation and amortization		298	299	597	598
Senior note and other interest expense		0	0	0	0
Income tax expense (benefit) in net income		220	92	350	186
Assets		46,510	46,756	46,510	46,756
All Other | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue	[1]	0	0	0	0
Operating expense	[1]	3,066	3,476	6,023	7,749
Operating income (loss)	[1]	(3,066)	(3,476)	(6,023)	(7,749)
Net income (loss)	[1]	(1,122)	(2,944)	(1,903)	(7,141)
Depreciation and amortization	[1]	0	0	0	0
Senior note and other interest expense	[1]	544	544	1,115	1,088
Income tax expense (benefit) in net income	[1]	(312)	(867)	(565)	(2,162)
Assets	[1]	413,587	(117)	413,587	(117)
SJW Group | Regulated
Segment Reporting Information [Line Items]
Operating revenue		99,773	95,798	174,920	168,151
Operating expense		75,564	70,642	136,149	132,343
Operating income (loss)		24,209	25,156	38,771	35,808
Net income (loss)		13,662	15,022	19,762	19,817
Depreciation and amortization		14,698	13,272	29,447	26,473
Senior note and other interest expense		6,170	5,540	11,390	11,048
Income tax expense (benefit) in net income		4,154	4,650	6,230	5,142
Assets		1,537,404	1,449,714	1,537,404	1,449,714
SJW Group | Non-tariffed
Segment Reporting Information [Line Items]
Operating revenue		3,192	3,288	5,727	5,977
Operating expense		5,430	5,645	10,119	11,655
Operating income (loss)		(2,238)	(2,357)	(4,392)	(5,678)
Net income (loss)		(124)	(2,151)	(351)	(5,661)
Depreciation and amortization		403	384	799	766
Senior note and other interest expense		544	544	1,115	1,088
Income tax expense (benefit) in net income		38	(582)	7	(1,654)
Assets		$ 466,246	$ 50,407	$ 466,246	$ 50,407

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