Balancing and Memorandum Accounts (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		$ 21,628	$ 19,276	$ 25,549	$ 21,706
Regulatory Asset Increase (Decrease)		(9,275)	3,691	(5,410)	1,251
Refunds (Collections) Adjustments		(7,658)	3	(15,444)	14
Ending Balance		4,695	22,971	4,695	22,971
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		6,549	7,250	8,864	6,220
Regulatory Asset Increase (Decrease)		1,250	2,527	(164)	3,556
Refunds (Collections) Adjustments		(886)	0	(1,787)	1
Ending Balance		6,913	9,777	6,913	9,777
Balancing and Memorandum Account [Roll Forward]
Beginning Balance		28,177	26,526	34,413	27,926
Regulatory Asset Increase (Decrease)		(8,025)	6,218	(5,574)	4,807
Refunds (Collections)		(8,544)	3	(17,231)	15
Ending Balance		11,608	32,748	11,608	32,748
Total balance, net under-collection amount		$ 1,183		$ 1,183
Authorized revenue, threshold percentage		2.00%		2.00%
2014-2017 WCMA
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance	[1]	$ 5,315	7,277	$ 7,750	6,679
Regulatory Asset Increase (Decrease)	[1]	0	128	0	723
Refunds (Collections) Adjustments	[1]	(2,397)	1	(4,832)	4
Ending Balance	[1]	2,918	7,406	2,918	7,406
2018 WCMA
Public Utilities, General Disclosures [Line Items]
Regulatory liability, due to sales in excess of authorized usage			4,660		8,070
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance	[1]	8,958	3,003	9,386	0
Regulatory Asset Increase (Decrease)	[1]	(8,958)	4,064	(9,386)	7,067
Refunds (Collections) Adjustments	[1]	0	0	0	0
Ending Balance	[1]	0	7,067	0	7,067
2019 WCMA
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance	[1]	557	0	0	0
Regulatory Asset Increase (Decrease)	[1]	(557)	0	0	0
Refunds (Collections) Adjustments	[1]	0	0	0	0
Ending Balance	[1]	0	0	0	0
2012 General Rate Case true-up
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		7,731	11,324	11,328	11,320
Regulatory Asset Increase (Decrease)		(1)	0	95	0
Refunds (Collections) Adjustments		(3,631)	2	(7,324)	6
Ending Balance		4,099	11,326	4,099	11,326
Cost of capital memorandum account
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		(1,540)	(1,507)	(1,523)	(144)
Regulatory Asset Increase (Decrease)		(7)	(8)	(24)	(1,371)
Refunds (Collections) Adjustments		0	0	0	0
Ending Balance		(1,547)	(1,515)	(1,547)	(1,515)
Tax memorandum account
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		(6,585)	(5,496)	(6,504)	0
Regulatory Asset Increase (Decrease)		(33)	(459)	(114)	(5,955)
Refunds (Collections) Adjustments		0	0	0	0
Ending Balance		(6,618)	(5,955)	(6,618)	(5,955)
All others
Balancing and Memorandum Revenue Account [Roll Forward]
Beginning Balance		7,192	4,675	5,112	3,851
Regulatory Asset Increase (Decrease)		281	(34)	4,019	787
Refunds (Collections) Adjustments		(1,630)	0	(3,288)	4
Ending Balance		5,843	4,642	5,843	4,642
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		870	0	1,090	0
Regulatory Asset Increase (Decrease)		2	0	8	0
Refunds (Collections) Adjustments		(223)	0	(449)	0
Ending Balance		649	0	649	0
Water supply costs
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		5,912	9,387	9,617	8,679
Regulatory Asset Increase (Decrease)		1,049	2,366	(754)	3,074
Refunds (Collections) Adjustments		(1,871)	0	(3,773)	0
Ending Balance		5,090	11,753	5,090	11,753
Pension
Balancing and Memorandum Cost Recovery Account [Roll Forward]
Beginning Balance		(233)	(2,137)	(1,843)	(2,459)
Regulatory Asset Increase (Decrease)		199	161	582	482
Refunds (Collections) Adjustments		1,208	0	2,435	1
Ending Balance		1,174	$ (1,976)	1,174	$ (1,976)
2018 WCMA Reserve Recorded
Public Utilities, General Disclosures [Line Items]
WCMA Reserve		1,003		1,003
2017 WCMA Reserve Recorded
Public Utilities, General Disclosures [Line Items]
WCMA Reserve		$ 985		$ 985

[1] As of September 30, 2019, the 2019 WCMA and 2018 WCMA balances were reserved in full. As of September 30, 2018, the reserve balances for the 2018 WCMA was $1,003 which has been included in the balance above. As of September 30, 2018, the reserve balance for the 2017 WCMA was $985 which has been included in the balance above.