Segment and Non-Tariffed Business Reporting (Details) $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) subsidiary reportable_segment		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Subsidiaries | subsidiary											4
Number of Reportable Segments | reportable_segment											2
Operating revenue		$ 125,838		$ 113,997	$ 102,965	$ 77,682	$ 98,718	$ 124,853	$ 99,086	$ 75,042	$ 420,482		$ 397,699	$ 389,225
Utilities Operating Expense											363,003		324,259	287,374
Operating Income (Loss)		6,016		17,084	21,971	12,408	17,482	25,828	22,799	7,331	57,479		73,440	101,851
SJW Group net income		(5,486)		$ 9,478	$ 13,538	$ 5,873	8,823	$ 15,788	$ 12,871	$ 1,285	23,403		38,767	59,204
Net income before noncontrolling interest											23,627		38,767	61,100
Depreciation and amortization											65,592		54,601	48,292
Interest Expense											31,796		24,332	22,929
Provision for income taxes											8,454		10,065	35,393
Assets		3,132,470					1,956,389				3,132,470		1,956,389	1,458,001
Regulated [Member] | Water Utility Services [Member]
Segment Reporting Information [Line Items]
Operating revenue											407,116	[1]	384,639	376,104
Utilities Operating Expense											329,520	[1]	294,536	276,061
Operating Income (Loss)											77,596	[1]	90,103	100,043
SJW Group net income											42,691	[1]	53,181	47,736
Depreciation and amortization											63,775	[1]	53,067	46,500
Interest Expense											25,073	[1]	22,157	20,670
Provision for income taxes											8,382	[1]	14,826	30,127
Assets		3,016,846	[1]				1,492,954				3,016,846	[1]	1,492,954	1,406,221
Regulated [Member] | SJW Corp. [Member]
Segment Reporting Information [Line Items]
Operating revenue											407,116		384,639	376,104
Utilities Operating Expense											329,520		294,536	276,061
Operating Income (Loss)											77,596		90,103	100,043
SJW Group net income											42,691		53,181	47,736
Depreciation and amortization											63,775		53,067	46,500
Interest Expense											25,073		22,157	20,670
Provision for income taxes											8,382		14,826	30,127
Assets		3,016,846					1,492,954				3,016,846		1,492,954	1,406,221
Nonregulated Operation [Member] | Water Utility Services [Member]
Segment Reporting Information [Line Items]
Operating revenue											7,969		7,578	7,419
Utilities Operating Expense											5,443		5,012	4,855
Operating Income (Loss)											2,526		2,566	2,564
SJW Group net income											2,019		1,848	1,137
Depreciation and amortization											420		338	572
Interest Expense											0		0	0
Provision for income taxes											708		719	993
Assets		5,507					4,489				5,507		4,489	4,471
Nonregulated Operation [Member] | Real Estate Services [Member]
Segment Reporting Information [Line Items]
Operating revenue											5,397		5,482	5,702
Utilities Operating Expense											3,751		3,539	3,688
Operating Income (Loss)											1,646		1,943	2,014
SJW Group net income											1,623		885	6,193
Depreciation and amortization											1,196		1,196	1,220
Interest Expense											0		0	60
Provision for income taxes											486		903	644
Assets		46,778					46,517				46,778		46,517	47,668
Nonregulated Operation [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	[2]										0		0	0
Utilities Operating Expense	[2]										24,289		21,172	2,770
Operating Income (Loss)	[2]										(24,289)		(21,172)	(2,770)
SJW Group net income	[2]										(22,930)		(17,147)	4,138
Depreciation and amortization	[2]										201		0	0
Interest Expense	[2]										6,723		2,175	2,199
Provision for income taxes	[2]										(1,122)		(6,383)	3,629
Assets	[2]	63,339					412,429				63,339		412,429	(359)
Nonregulated Operation [Member] | SJW Corp. [Member]
Segment Reporting Information [Line Items]
Operating revenue											13,366		13,060	13,121
Utilities Operating Expense											33,483		29,723	11,313
Operating Income (Loss)											(20,117)		(16,663)	1,808
SJW Group net income											(19,288)		(14,414)	11,468
Depreciation and amortization											1,817		1,534	1,792
Interest Expense											6,723		2,175	2,259
Provision for income taxes											72		(4,761)	5,266
Assets		$ 115,624					$ 463,435				$ 115,624		$ 463,435	$ 51,780

[1] As of December 31, 2019, the Company has performed a preliminary allocation of goodwill associated with the acquisition of CTWS to two reporting units, Connecticut and Maine, which are both aggregated within the Regulated Water Utility Services reportable segment.

[2] The “All Other” category for the year ended December 31, 2019, includes the accounts of SJW Group, SJWNE, LLC and CTWS, Inc. on a stand-alone basis. For the years ended December 31, 2018 and 2017, the “All Other” category includes the accounts of SJW Group on a stand-alone basis.