Summary of Significant Accounting Policies (Balancing and Memorandum Accounts) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Surcharge Offset			$ (7,493)	$ (16,708)	$ (12,139)
Memorandum Account, Net1			11,663	4,357	8,910	$ (1,377)
Memorandum Account, Revenue Increase (Reduction)			19,050	18,863	21,986
Memorandum Account, Refunds (Collections)			(4,251)	(6,708)	440
Balancing Account, Beginning Balance	$ 21,552	$ 21,552	21,552	36,296	48,438
Balancing Account, Revenue Increase (Reduction)			3,137	11,024	2,745
Balancing Account, Refunds (Collections)			(16,955)	(42,476)	(27,026)
Balancing Account, Surcharge Offset			8,528	16,708	12,139
Balancing Account, Ending Balance			16,262	21,552	36,296
Balancing and Memorandum Account, Beginning Balance	25,909	25,909	25,909	45,206	47,061
Balancing and Memorandum Account, Revenue Increase (Reduction), Net			22,187	29,887	24,731
Balancing and Memorandum Account, Refunds (Collections), Net			(21,206)	(49,184)	(26,586)
Balancing and Memorandum Account, Surcharge Offset			1,035	0	0
Balancing and Memorandum Account, Ending Balance			27,925	25,909	45,206
2017 WCMA Reserve Recorded [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Revenue Increase (Reduction)			1,169
2014 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
WCMA Reserve			0	1,090	1,278
Memorandum Account, Surcharge Offset			0	0	0
Memorandum Account, Net1			43	0	2,944	0
Memorandum Account, Revenue Increase (Reduction)			1,090	188	2,981
Memorandum Account, Refunds (Collections)			(1,047)	(3,132)	(37)
2015 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
WCMA Reserve			0	2,115	2,343
Memorandum Account, Surcharge Offset			0	0	(12,139)
Memorandum Account, Net1			33	1,589	5,372	0
Memorandum Account, Revenue Increase (Reduction)			2,101	211	17,511
Memorandum Account, Refunds (Collections)			(3,657)	(3,994)	0
Balancing and Memorandum Account, Surcharge Offset					12,139
2016 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Surcharge Offset			(1,452)	(16,708)
Memorandum Account, Net1			115	0	0
Memorandum Account, Revenue Increase (Reduction)	1,371		1,567	16,708
Memorandum Account, Surcharge Offset, Increase for Interest			196
Memorandum Account, Refunds (Collections)			0	0
2017 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Revenue Increase (Reduction), Gross			13,699
Memorandum Account, Surcharge Offset			(6,041)
Memorandum Account, Net1			6,489	0
Memorandum Account, Revenue Increase (Reduction)		961	12,530
Memorandum Account, Refunds (Collections)			0
2017 WCMA Post Drought Surcharges [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Revenue Increase (Reduction)			7,658
Water supply balancing accounts [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	5,190	5,190	5,190	2,771	890
Balancing Account, Revenue Increase (Reduction)			2,921	1,620	2,025
Balancing Account, Refunds (Collections)			568	799	(144)
Balancing Account, Surcharge Offset			0	0	0
Balancing Account, Ending Balance			8,679	5,190	2,771
Drought Surcharges [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	(7,688)	(7,688)	(7,688)	(359)	0
Balancing Account, Revenue Increase (Reduction)			0	0	0
Balancing Account, Refunds (Collections)			(765)	(24,037)	(12,498)
Balancing Account, Surcharge Offset			8,453	16,708	12,139
Balancing Account, Ending Balance			0	(7,688)	(359)
Pension Balancing Account [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	(2,009)	(2,009)	(2,009)	(552)	1,412
Balancing Account, Revenue Increase (Reduction)			894	1,120	(924)
Balancing Account, Refunds (Collections)			(1,344)	(2,577)	(1,040)
Balancing Account, Surcharge Offset			0	0	0
Balancing Account, Ending Balance			(2,459)	(2,009)	(552)
2012 General Rate Case True-up [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	20,682	20,682	20,682	33,070	44,400
Balancing Account, Revenue Increase (Reduction)			0	0	1,937
Balancing Account, Refunds (Collections)			(9,363)	(12,388)	(13,267)
Balancing Account, Surcharge Offset			0	0	0
Balancing Account, Ending Balance			11,319	20,682	33,070
2015 General Rate Case True-up [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	5,528	5,528	5,528	0
Balancing Account, Revenue Increase (Reduction)			0	8,767
Balancing Account, Refunds (Collections)			(5,413)	(3,239)
Balancing Account, Surcharge Offset			0	0
Balancing Account, Ending Balance			115	5,528	0
Other Regulatory Assets [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Surcharge Offset			0	0	0
Memorandum Account, Net1			4,983	2,768	594	$ (1,377)
Memorandum Account, Revenue Increase (Reduction)			1,762	1,756	1,494
Memorandum Account, Refunds (Collections)			453	418	477
Balancing Account, Beginning Balance	$ (151)	$ (151)	(151)	1,366	1,736
Balancing Account, Revenue Increase (Reduction)			(678)	(483)	(293)
Balancing Account, Refunds (Collections)			(638)	(1,034)	(77)
Balancing Account, Surcharge Offset			75	0	0
Balancing Account, Ending Balance			$ (1,392)	$ (151)	$ 1,366