SJW Group and CTWS Merger (the “Merger”) (Details) $ / shares in Units, $ in Thousands					3 Months Ended	12 Months Ended
	Oct. 09, 2019 USD ($) $ / shares	Oct. 08, 2019 USD ($)	Dec. 05, 2018 shares	Dec. 03, 2018 shares	Dec. 31, 2019 USD ($) people_served service_connection communities_served	Dec. 31, 2019 USD ($) people_served service_connection communities_served $ / shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Share-based compensation related to business combination						$ 6,384
Proceeds from debt financing						590,000	$ 0	$ 0
Borrowings from lines of credit						105,349	76,000	$ 56,500
Goodwill					$ 628,287	628,287	1,680
Total revenues						515,153	514,364
Pretax income (loss)						60,862	72,938
Net income (loss)						$ 56,968	$ 65,449
Basic earnings per share (in dollars per share) | $ / shares						$ 2.00	$ 2.31
CTWS [Member]
Business Acquisition [Line Items]
Acquiree's common shares converted in cash (in USD per share) | $ / shares	$ 70.00
Payments to Acquire Businesses, Gross	$ 838,475
Cash Acquired from Acquisition	3,011
Share-based compensation related to business combination	6,384
Proceeds from debt financing	18,463
Borrowings from lines of credit	60,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Nonutility Plant, Net	848
Utility plant, net	750,703
Nonutility plant	42,673
Current assets	12,489
Regulatory assets and deferred charges, less current portion	83,132
Other intangible assets	17,181
Other assets	2,592
Goodwill	626,523
Total assets acquired	1,536,141
Long-term debt	281,009
Current liabilities, including maturities of long-term debt	125,772
Deferred income taxes	107,789
Post-retirement benefit plans	31,789
Contributions in aid of construction and construction advances	137,327
Other long-term liabilities	10,607
Total liabilities assumed	694,293
Assumed equity	$ 841,848
Total revenues					21,672
Pretax (loss)					$ (3,174)
Connecticut Water Service, Inc. [Member]
Business Acquisition [Line Items]
Number of people served | people_served					480,000	480,000
Public Utilities, Number of Communities Served | communities_served					80	80
Common Stock [Member] | Underwritten Public Offering [Member]
Business Acquisition [Line Items]
Common stock issued, shares | shares			1,012,500	6,750,000			7,762,000
Proceeds from Issuance of Common Stock, Gross							$ 411,077
Series 2019A/B/C [Member] | Senior Notes [Member] | SJW Group [Member]
Business Acquisition [Line Items]
Principal Debt Sold		$ 427,398
Connecticut and Maine | Connecticut Water Service, Inc. [Member]
Business Acquisition [Line Items]
Public Utilities, Service Connections | service_connection					137,000	137,000
Connecticut | Connecticut Water Service, Inc. [Member]
Business Acquisition [Line Items]
Public Utilities, Service Connections | service_connection					3,000	3,000