Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 134,254	$ 112,812
Maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized	24 months
Regulatory balancing account revenue	$ (2,007)	9,224
Operating revenue	132,247	122,036
Lease revenue	511	507
Non-regulated revenue	4,419	3,462
Cash and cash equivalents			$ 34,702	$ 94,776	$ 12,693	$ 25,935
Restricted cash (included in taxes, prepaid expenses and other assets)			631	576
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	95,352	25,935	$ 35,333	$ 95,352	12,693	25,935
Change in restricted cash		0
Net cash used in investing activities	(70,650)	(52,239)
Change in cash, cash equivalents, and restricted cash	(60,019)	(13,242)
Cash, cash equivalents, and restricted cash at beginning of period	95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	35,333	12,693
Administrative and general	26,319	22,746
Total operating expenses	124,194	111,483
Net operating income	8,053	10,553
Other components of net periodic benefit cost	(2,546)	(2,503)
Net other income	(1,895)	(1,205)
As Reported on Form 10-Q
Summary of Significant Accounting Policies [Line Items]
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows		25,492			11,990	25,492
Change in restricted cash		(260)
Net cash used in investing activities		(52,499)
Change in cash, cash equivalents, and restricted cash		(13,502)
Cash, cash equivalents, and restricted cash at beginning of period		25,492
Cash, cash equivalents, and restricted cash at end of period		11,990
Administrative and general		25,249
Total operating expenses		113,986
Net operating income		8,050
Other components of net periodic benefit cost		0
Net other income		1,298
Increase (Decrease) from Retrospective Adoption | Accounting Standards Update 2016-18
Summary of Significant Accounting Policies [Line Items]
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows		443			$ 703	$ 443
Change in restricted cash		260
Net cash used in investing activities		260
Change in cash, cash equivalents, and restricted cash		260
Cash, cash equivalents, and restricted cash at beginning of period		443
Cash, cash equivalents, and restricted cash at end of period		703
Increase (Decrease) from Retrospective Adoption | Accounting Standards Update 2017-07
Summary of Significant Accounting Policies [Line Items]
Administrative and general		(2,503)
Total operating expenses		(2,503)
Net operating income		2,503
Other components of net periodic benefit cost		2,503
Net other income		(2,503)
Residential
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	91,319	75,865
Business
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	27,057	22,026
Industrial
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	7,579	6,954
Public authorities
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	5,444	4,146
Other
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	2,855	3,821
Regulated water and wastewater services
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	134,254	112,812
Operating revenue	132,247	122,036
Operating and maintenance revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	3,165	1,913
Other non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	743	1,042
Non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	3,908	2,955
Non-regulated revenue	$ 4,419	$ 3,462