Capitalization	12 Months Ended
Dec. 31, 2020
Equity [Abstract]
Capitalization	CapitalizationSJW Group is authorized to issue 70,000,000 shares of common stock of $0.001 par value per share. At December 31, 2020 and 2019, 28,556,605 and 28,456,508, respectively, shares of common stock were issued and outstanding.As of December 31, 2020 and 2019, 1,000,000 shares of preferred stock of $0.001 par value per share were authorized for SJW Group. At December 31, 2020 and 2019, no shares of preferred stock were issued or outstanding.

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e)(1))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-URI http://asc.fasb.org/topic&trid=2208762

Name:	us-gaap_StockholdersEquityNoteDisclosureTextBlock
Namespace Prefix:	us-gaap_
Data Type:	nonnum:textBlockItemType
Balance Type:	na
Period Type:	duration