Balancing and Memorandum Account Recovery Procedures (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		$ 19,676	$ 24,067	$ 21,706	$ 25,909
Regulatory Asset Increase (Decrease)		(408)	10,252	(2,438)	14,587
Refunds (Collections)		8	(5,303)	8	(11,556)
Surcharge Offset		0	0	0	76
Ending Balance		19,276	29,016	19,276	29,016
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		5,899	0	6,220	0
Regulatory Asset Increase (Decrease)		1,351	0	1,030	0
Refunds (Collections)		0	0	0	0
Surcharge Offset		0	0	0	0
Ending Balance		7,250	0	7,250	0
Balancing and Memorandum Account [Roll Forward]
Beginning Balance		25,575	24,067	27,926	25,909
Regulatory Asset Increase (Decrease)		943	10,252	(1,408)	14,587
Refunds (Collections)		8	(5,303)	8	(11,556)
Surcharge Offset		0	0	0	76
Ending Balance		26,526	29,016	26,526	29,016
Regulatory Balancing and Memorandum Accounts, Net Under-Collected		3,900		3,900
2014-2016 WCMA
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		270	398	190	1,589
Regulatory Asset Increase (Decrease)		93	3,206	173	4,654
Refunds (Collections)		2	(556)	2	(1,788)
Surcharge Offset		0	(45)	0	(1,452)
Ending Balance		365	3,003	365	3,003
2017 WCMA
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance	[1]	6,785	0	6,489	0
Regulatory Asset Increase (Decrease)	[1]	127	4,989	423	7,049
Refunds (Collections)	[1]	0	0	0	0
Surcharge Offset	[1]	0	(3,988)	0	(6,048)
Ending Balance	[1]	6,912	1,001	6,912	1,001
2018 WCMA
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance	[1]	(708)	0	0	0
Regulatory Asset Increase (Decrease)	[1]	3,711	0	3,003	0
Refunds (Collections)	[1]	0	0	0	0
Surcharge Offset	[1]	0	0	0	0
Ending Balance	[1]	3,003	0	3,003	0
Balancing and Memorandum Account [Roll Forward]
Memorandum Account, Revenue Increase (Reduction)		(4,118)		(3,410)
2012 General Rate Case true-up
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		11,320	18,424	11,320	20,682
Regulatory Asset Increase (Decrease)		0	0	0	0
Refunds (Collections)		4	(2,659)	4	(4,917)
Surcharge Offset		0	0	0	0
Ending Balance		11,324	15,765	11,324	15,765
2015 General Rate Case true-up
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		115	4,097	115	5,528
Regulatory Asset Increase (Decrease)		0	0	0	0
Refunds (Collections)		2	(1,686)	2	(3,117)
Surcharge Offset		0	0	0	0
Ending Balance		117	2,411	117	2,411
Cost of capital
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		(1,309)	(459)	(144)	(817)
Regulatory Asset Increase (Decrease)		(198)	0	(1,363)	0
Refunds (Collections)		0	315	0	673
Surcharge Offset		0	0	0	0
Ending Balance		(1,507)	(144)	(1,507)	(144)
Tax memorandum
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		(933)	0	0	0
Regulatory Asset Increase (Decrease)		(4,563)	0	(5,496)	0
Refunds (Collections)		0	0	0	0
Surcharge Offset		0	0	0	0
Ending Balance		(5,496)	0	(5,496)	0
Drought surcharges
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		0	(5,054)	0	(7,688)
Regulatory Asset Increase (Decrease)		0	0	0	0
Refunds (Collections)		0	60	0	(773)
Surcharge Offset		0	4,033	0	7,500
Ending Balance		0	(961)	0	(961)
Cost-Recovery Accounts [Member]
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		0	3,145	0	3,181
Regulatory Asset Increase (Decrease)		0	1,631	0	2,001
Refunds (Collections)		0	(369)	0	(775)
Surcharge Offset		0	0	0	0
Ending Balance		0	4,407	0	4,407
Water supply costs
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		8,197	0	8,679	0
Regulatory Asset Increase (Decrease)		1,190	0	708	0
Refunds (Collections)		0	0	0	0
Surcharge Offset		0	0	0	0
Ending Balance		9,387	0	9,387	0
Pension
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		(2,298)	0	(2,459)	0
Regulatory Asset Increase (Decrease)		161	0	322	0
Refunds (Collections)		0	0	0	0
Surcharge Offset		0	0	0	0
Ending Balance		(2,137)	0	(2,137)	0
All others
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		4,136	3,516	3,736	3,434
Regulatory Asset Increase (Decrease)		422	426	822	883
Refunds (Collections)		0	(408)	0	(859)
Surcharge Offset		0	0	0	76
Ending Balance		4,558	$ 3,534	4,558	3,534
2017 WCMA Reserve Recorded
Balancing and Memorandum Account [Roll Forward]
Memorandum Account, Revenue Increase (Reduction)				(938)	$ (276)
2018 WCMA Reserve Recorded
Balancing and Memorandum Account [Roll Forward]
Memorandum Account, Revenue Increase (Reduction)		$ (407)		$ (407)

[1] As of June 30, 2018, the reserve balances for the 2017 and 2018 WCMA were $938 and $407, respectively, which have been netted from the balances above. As of June 30, 2017, the reserve balance for the 2017 WCMA was $276 which has been netted from the balance above.