Financial Statement Schedule - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net income before noncontrolling interest	$ 61,515	$ 23,627	$ 38,767
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income taxes	(5,497)	(1,249)	(9,486)
Share-based compensation	3,553	3,406	2,117
Other non-cash, net	0	20	414
Changes in operating assets and liabilities, net of acquired assets and liabilities:
Accounts payable and other current liabilities	(357)	1,946	2,130
Tax receivable and accrued taxes	(3,557)	(6,044)	5,841
Other changes, net	(2,413)	7,469	1,706
Net cash provided by operating activities	104,051	130,005	91,343
Investing activities:
Payments for business/asset acquisition and water rights	0	(835,465)	(2,496)
Proceeds from sale of equity investments	0	0	4,112
Net cash used in investing activities	(214,259)	(1,017,771)	(146,843)
Financing activities:
Long-term borrowings	85,000	590,000	0
Issuance of common stock, net of issuance costs	0	(10)	411,385
Debt issuance costs	(829)	(4,918)	0
Dividends paid	(36,509)	(34,134)	(23,074)
Other changes, net	(190)	1,163	(3,078)
Net cash provided by financing activities	101,533	484,988	468,423
Net change in cash, cash equivalents and restricted cash	(8,675)	(402,778)	412,923
Cash, cash equivalents and restricted cash, beginning of year	17,944	420,722	7,799
Cash, cash equivalents and restricted cash, end of year	9,269	17,944	420,722
Cash paid during the year for:
Interest	59,955	32,138	27,038
Income taxes	10,380	16,448	13,750
SJW Group
Adjustments to reconcile net income to net cash provided by operating activities:
Earnings from investment in subsidiaries	(78,141)	(45,191)	(55,379)
Deferred income taxes	162	4,606	(5,565)
Share-based compensation	466	141	44
Other non-cash, net	0	0	423
Changes in operating assets and liabilities, net of acquired assets and liabilities:
Accounts payable and other current liabilities	(1,532)	255	1,039
Intercompany receivables	(2,607)	2,045	209
Tax receivable and accrued taxes	10,759	(6,779)	(5,592)
Accrued interest	(553)	3,742	0
Return on capital from investments in subsidiaries	47,800	28,600	20,750
Other changes, net	367	808	36
Net cash provided by operating activities	38,236	11,630	(5,268)
Investing activities:
Proceeds to subsidiaries for notes receivable	(10,353)	(102,522)	(2,030)
Repayments from subsidiaries for notes receivable	1,400	95,795	17,458
Investments in subsidiaries	(37,952)	(62,262)	0
Payments for business/asset acquisition and water rights	0	(837,903)	0
Proceeds from sale of equity investments	0	0	4,112
Net cash used in investing activities	(46,905)	(906,892)	19,540
Financing activities:
Borrowings from subsidiaries for notes payable	54,837	29,865	12,705
Repayments to subsidiaries for notes payable	(59,762)	(17,705)	(3,358)
Long-term borrowings	50,000	510,000	0
Issuance of common stock, net of issuance costs	0	0	411,385
Debt issuance costs	(496)	(3,957)	0
Dividends paid	(36,509)	(34,134)	(23,074)
Other changes, net	0	(10)	0
Net cash provided by financing activities	8,070	484,059	397,658
Net change in cash, cash equivalents and restricted cash	(599)	(411,203)	411,930
Cash, cash equivalents and restricted cash, beginning of year	1,035	412,238	308
Cash, cash equivalents and restricted cash, end of year	436	1,035	412,238
Cash paid during the year for:
Interest	19,442	2,203	2,175
Income taxes	3,466	689	824
Supplemental disclosure of non-cash activities:
Share-based compensation from investment in subsidiaries	$ 2,894	$ 4,663	$ (714)