Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 209,541	$ 197,007	$ 515,567	$ 472,412
Regulatory balancing account revenue	9,442	14,724	8,295	32,487
Total operating revenue	218,983	211,731	$ 523,862	504,899
Maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized			24 months
Total non-regulated revenue	4,703	3,542	$ 13,967	10,743
Cash and cash equivalents					$ 66,421	$ 94,776
Restricted cash (included in taxes, prepaid expenses and other assets)					671	576
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	67,092	29,463	95,352	25,935	$ 67,092	$ 95,352
Change in restricted cash				0
Net cash used in investing activities			(214,290)	(182,832)
Change in cash, cash equivalents, and restricted cash			(28,260)	3,528
Cash, cash equivalents, and restricted cash at beginning of period			95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	67,092	29,463	67,092	29,463
Administrative and general	26,493	22,749	77,195	66,908
Income taxes	11,262	18,219	15,380	27,941
Total operating expenses		169,269		425,213
Net operating income	45,548	42,462	78,657	79,686
Other components of net periodic benefit cost	(1,975)	(2,137)	(6,984)	(7,023)
Income tax benefit (expense) on other income and expenses	305	30	1,882	(1,105)
Net other income (loss)	(841)	(36)	(4,940)	(866)
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		28,341		25,492
Change in restricted cash				(679)
Net cash used in investing activities				(183,511)
Change in cash, cash equivalents, and restricted cash				2,849
Cash, cash equivalents, and restricted cash at beginning of period				25,492
Cash, cash equivalents, and restricted cash at end of period		28,341		28,341
Administrative and general		24,886		73,931
Income taxes		17,348		26,099
Total operating expenses		170,535		430,394
Net operating income		41,196		74,505
Other components of net periodic benefit cost		0		0
Income tax benefit (expense) on other income and expenses		(841)		(2,947)
Net other income (loss)		1,230		4,315
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		1,122		443
Change in restricted cash				679
Net cash used in investing activities				679
Change in cash, cash equivalents, and restricted cash				679
Cash, cash equivalents, and restricted cash at beginning of period				443
Cash, cash equivalents, and restricted cash at end of period		1,122		1,122
Increase (Decrease) from Retrospective Adoption | Accounting Standards Update 2017-07
Summary of Significant Accounting Policies [Line Items]
Administrative and general		(2,137)		(7,023)
Income taxes		871		1,842
Total operating expenses		(1,266)		(5,181)
Net operating income		1,266		5,181
Other components of net periodic benefit cost		2,137		7,023
Income tax benefit (expense) on other income and expenses		871		1,842
Net other income (loss)		(1,266)		(5,181)
Regulated water and wastewater services
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	209,541	197,007	515,567	472,412
Total operating revenue	218,983	211,731	523,862	504,899
Operating and maintenance revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	2,816	1,903	8,278	5,870
Other non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	1,328	1,143	4,053	3,362
Non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	4,144	3,046	12,331	9,232
Lease revenue	559	496	1,636	1,511
Total non-regulated revenue	4,703	3,542	13,967	10,743
Residential
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	138,939	131,608	340,107	309,645
Business
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	38,538	35,678	97,720	87,246
Industrial
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	8,987	7,919	24,507	21,281
Public authorities
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	12,180	11,480	25,875	23,500
Other
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 10,897	$ 10,322	$ 27,358	$ 30,740