SJW Group and Connecticut Water Service Merger Agreement (Details) - $ / shares	Mar. 14, 2018	Sep. 30, 2018	Dec. 31, 2017
Business Combinations [Abstract]
Conversion rate of CTWS shares to SJW Group (in shares)	70.00
SJW Group common stock, par value (usd per share)		$ 0.001	$ 0.001

Number of shares of equity interests issued or issuable to acquire entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration