Summary of Significant Accounting Policies (Balancing and Memorandum Accounts) (Details) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	$ 25,549,000	$ 25,549,000	$ 21,705,000
Balancing and Memorandum Revenue Account, Increase (Decrease)		(4,984,000)	3,827,000
Balancing and Memorandum Revenue Account, Refunds (Collections)		(22,505,000)	17,000
Balancing and Memorandum Revenue Account, Surcharge Offset		0	0
Revenue Accounts, Ending Balance		(1,940,000)	25,549,000	$ 21,705,000
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Cost-recovery Accounts, Beginning Balance	8,864,000	8,864,000	6,220,000
Balancing and Memorandum Cost Recovery Account, Increase (Decrease)		962,000	2,643,000
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)		(2,603,000)	1,000
Balancing and Memorandum Cost Recovery Account, Surcharge Offset		0	0
Cost-recovery Accounts, Ending Balance		7,223,000	8,864,000	6,220,000
Balancing and Memorandum Account [Roll Forward]
Balancing and Memorandum Account, Beginning Balance	34,413,000	34,413,000	27,925,000	25,909,000
Balancing and Memorandum Account, Increase (Decrease)		(4,022,000)	6,470,000	22,187,000
Balancing and Memorandum Account, Refunds (Collections), Net		(25,108,000)	18,000	(21,206,000)
Balancing and Memorandum Account, Surcharge Offset		0	0	1,035,000
Balancing and Memorandum Account, Ending Balance		5,283,000	34,413,000	27,925,000
Regulatory Balancing and Memorandum Accounts, Net Under-Collected		$ 1,428,000
Regulatory Balancing and Memorandum Accounts Review, Authorized Revenue, Threshold Percentage		2.00%
2017 WCMA [Member]
Balancing and Memorandum Account [Roll Forward]
Balancing and Memorandum Revenue Account, Increase (Decrease)			1,182
Balancing and Memorandum Revenue Account, Surcharge Offset			0
2014-2017 WCMA [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	7,750,000	$ 7,750,000	6,680,000	1,589,000
Balancing and Memorandum Revenue Account, Increase (Decrease)		0	1,066,000	17,288,000
Balancing and Memorandum Revenue Account, Refunds (Collections)		(7,042,000)	4,000	(4,704,000)
Balancing and Memorandum Revenue Account, Surcharge Offset				(7,493,000)
Revenue Accounts, Ending Balance		708,000	7,750,000	6,680,000
2017 WCMA Reserve Recorded [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	0	0
Balancing and Memorandum Revenue Account, Increase (Decrease)			1,169
Revenue Accounts, Ending Balance			0
2018 WCMA [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	9,386,000	9,386,000	0
Balancing and Memorandum Revenue Account, Increase (Decrease)		(9,386,000)	9,386,000
Balancing and Memorandum Revenue Account, Refunds (Collections)		0	0
Balancing and Memorandum Revenue Account, Surcharge Offset		0
Revenue Accounts, Ending Balance		0	9,386,000	0
2012 General Rate Case True-up [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	11,328,000	11,328,000	11,319,000	20,682,000
Balancing and Memorandum Revenue Account, Increase (Decrease)		96,000	0	0
Balancing and Memorandum Revenue Account, Refunds (Collections)		(10,672,000)	9,000	(9,363,000)
Balancing and Memorandum Revenue Account, Surcharge Offset		0	0	0
Revenue Accounts, Ending Balance		752,000	11,328,000	11,319,000
Cost of capital [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	(1,523,000)	(1,523,000)	(144,000)	(817,000)
Balancing and Memorandum Revenue Account, Increase (Decrease)		(30,000)	(1,379,000)	0
Balancing and Memorandum Revenue Account, Refunds (Collections)		0	0	673,000
Balancing and Memorandum Revenue Account, Surcharge Offset		0	0	0
Revenue Accounts, Ending Balance		(1,553,000)	(1,523,000)	(144,000)
Tax memorandum [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	(6,504,000)	(6,504,000)	0
Balancing and Memorandum Revenue Account, Increase (Decrease)		(139,000)	(6,504,000)
Balancing and Memorandum Revenue Account, Refunds (Collections)		0	0
Balancing and Memorandum Revenue Account, Surcharge Offset		0	0
Revenue Accounts, Ending Balance		(6,643,000)	(6,504,000)	0
Water supply balancing accounts [Member]
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Cost-recovery Accounts, Beginning Balance	9,617,000	9,617,000	8,679,000
Balancing and Memorandum Cost Recovery Account, Increase (Decrease)		207,000	939,000
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)		(5,496,000)	(1,000)
Balancing and Memorandum Cost Recovery Account, Surcharge Offset		0	0
Cost-recovery Accounts, Ending Balance		4,328,000	9,617,000	8,679,000
Pension Balancing Account [Member]
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Cost-recovery Accounts, Beginning Balance	(1,843,000)	(1,843,000)	(2,459,000)
Balancing and Memorandum Cost Recovery Account, Increase (Decrease)		745,000	614,000
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)		3,547,000	2,000
Balancing and Memorandum Cost Recovery Account, Surcharge Offset		0	0
Cost-recovery Accounts, Ending Balance		2,449,000	(1,843,000)	(2,459,000)
Other Regulatory Assets [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance	5,112,000	5,112,000	3,850,000	8,962,000
Balancing and Memorandum Revenue Account, Increase (Decrease)		4,475,000	1,258,000	1,084,000
Balancing and Memorandum Revenue Account, Refunds (Collections)		(4,791,000)	4,000	(6,271,000)
Balancing and Memorandum Revenue Account, Surcharge Offset		0	0	75,000
Revenue Accounts, Ending Balance		4,796,000	5,112,000	3,850,000
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Cost-recovery Accounts, Beginning Balance	1,090,000	1,090,000	0
Balancing and Memorandum Cost Recovery Account, Increase (Decrease)		10,000	1,090,000
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)		(654,000)	0
Balancing and Memorandum Cost Recovery Account, Surcharge Offset		0
Cost-recovery Accounts, Ending Balance		$ 446,000	1,090,000	0
Drought Surcharges [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance			0	(7,688,000)
Balancing and Memorandum Revenue Account, Increase (Decrease)				0
Balancing and Memorandum Revenue Account, Refunds (Collections)				(765,000)
Balancing and Memorandum Revenue Account, Surcharge Offset				8,453,000
Revenue Accounts, Ending Balance				0
Cost-Recovery Accounts [Member]
Balancing and Memorandum Account [Roll Forward]
Revenue Accounts, Beginning Balance			$ 6,220,000	3,181,000
Balancing and Memorandum Revenue Account, Increase (Decrease)				3,815,000
Balancing and Memorandum Revenue Account, Refunds (Collections)				(776,000)
Revenue Accounts, Ending Balance				$ 6,220,000
San Jose Water Company [Member] | Advice Letter No. 528/528A [Member]
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Balancing and Memorandum Cost Recovery Account, Refunds (Collections)	27,045,000
Balancing and Memorandum Account [Roll Forward]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 16,378,000