Summary of Significant Accounting Policies (Balancing and Memorandum Accounts) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Beginning Balance	$ 8,910	$ (1,377)	$ (1,896)
Memorandum Account, Revenue Increase (Reduction)	18,863	21,986	341
Memorandum Account, Refunds (Collections)	(6,708)	440	178
Memorandum Account, Surcharge Offset	(16,708)	(12,139)	0
Memorandum Account, Ending Balance	4,357	8,910	(1,377)
Balancing Account, Beginning Balance	36,296	48,438	9,585
Balancing Account, Revenue Increase (Reduction)	11,024	2,745	41,657
Balancing Account, Refunds (Collections)	(42,476)	(27,026)	(2,804)
Balancing Account, Surcharge Offset	16,708	12,139	0
Balancing Account, Ending Balance	21,552	36,296	48,438
Balancing and Memorandum Account, Beginning Balance	45,206	47,061	7,689
Balancing and Memorandum Account, Revenue Increase (Reduction), Net	29,887	24,731	41,998
Balancing and Memorandum Account, Refunds (Collections), Net	(49,184)	(26,586)	(2,626)
Balancing and Memorandum Account, Surcharge Offset	0	0	0
Balancing and Memorandum Account, Ending Balance	25,909	45,206	47,061
2014 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Beginning Balance	2,944	0
Memorandum Account, Revenue Increase (Reduction)	188	2,981
Memorandum Account, Refunds (Collections)	(3,132)	(37)
Memorandum Account, Surcharge Offset	0	0
Memorandum Account, Ending Balance	0	2,944	0
2015 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Beginning Balance	5,372	0
Memorandum Account, Revenue Increase (Reduction)	211	17,511
Memorandum Account, Refunds (Collections)	(3,994)	0
Memorandum Account, Surcharge Offset	0	(12,139)
Memorandum Account, Ending Balance	1,589	5,372	0
Balancing and Memorandum Account, Surcharge Offset		12,139
2016 WCMA [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Beginning Balance	0
Memorandum Account, Revenue Increase (Reduction)	16,708
Memorandum Account, Refunds (Collections)	0
Memorandum Account, Surcharge Offset	(16,708)
Memorandum Account, Ending Balance	0	0
Water supply balancing accounts [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	2,771	890	(2,378)
Balancing Account, Revenue Increase (Reduction)	1,620	2,025	3,353
Balancing Account, Refunds (Collections)	799	(144)	(85)
Balancing Account, Surcharge Offset	0	0	0
Balancing Account, Ending Balance	5,190	2,771	890
Drought Surcharges [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	(359)	0
Balancing Account, Revenue Increase (Reduction)	0	0
Balancing Account, Refunds (Collections)	(24,037)	(12,498)
Balancing Account, Surcharge Offset	16,708	12,139
Balancing Account, Ending Balance	(7,688)	(359)	0
Pension Balancing Account [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	(552)	1,412	9,734
Balancing Account, Revenue Increase (Reduction)	1,120	(924)	(7,705)
Balancing Account, Refunds (Collections)	(2,577)	(1,040)	(617)
Balancing Account, Surcharge Offset	0	0	0
Balancing Account, Ending Balance	(2,009)	(552)	1,412
2012 General Rate Case True-up [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	33,070	44,400	0
Balancing Account, Revenue Increase (Reduction)	0	1,937	46,456
Balancing Account, Refunds (Collections)	(12,388)	(13,267)	(2,056)
Balancing Account, Surcharge Offset	0	0	0
Balancing Account, Ending Balance	20,682	33,070	44,400
2015 General Rate Case True-up [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Balancing Account, Beginning Balance	0
Balancing Account, Revenue Increase (Reduction)	8,767
Balancing Account, Refunds (Collections)	(3,239)
Balancing Account, Surcharge Offset	0
Balancing Account, Ending Balance	5,528	0
Other Regulatory Assets [Member]
Schedule of Balancing and Memorandum Accounts [Line Items]
Memorandum Account, Beginning Balance	594	(1,377)	(1,896)
Memorandum Account, Revenue Increase (Reduction)	1,756	1,494	341
Memorandum Account, Refunds (Collections)	418	477	178
Memorandum Account, Surcharge Offset	0	0	0
Memorandum Account, Ending Balance	2,768	594	(1,377)
Balancing Account, Beginning Balance	1,366	1,736	2,229
Balancing Account, Revenue Increase (Reduction)	(483)	(293)	(447)
Balancing Account, Refunds (Collections)	(1,034)	(77)	(46)
Balancing Account, Surcharge Offset	0	0	0
Balancing Account, Ending Balance	$ (151)	$ 1,366	$ 1,736

Schedule of Balancing and Memorandum Accounts [Line Items]