Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 171,772	$ 162,593	$ 306,026	$ 275,405
Regulatory balancing account revenue	860	8,539	(1,147)	17,763
Total operating revenue	172,632	171,132	$ 304,879	293,168
Maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized			24 months
Lease revenue	567	508	$ 1,078	1,015
Total non-regulated revenue	4,845	3,739	9,264	7,201
Cash and cash equivalents					$ 33,668	$ 94,776	$ 30,144	$ 25,935
Restricted cash (included in taxes, prepaid expenses and other assets)					597	576
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	34,265	30,144	95,352	25,935	$ 34,265	$ 95,352	30,144	25,935
Change in restricted cash				0
Net cash used in investing activities			(133,900)	(109,478)
Change in cash, cash equivalents, and restricted cash			(61,087)	4,209
Cash, cash equivalents, and restricted cash at beginning of period			95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	34,265	30,144	34,265	30,144
Administrative and general	24,383	21,413	50,702	44,159
Income taxes	4,347	10,606	4,118	9,722
Total operating expenses		144,461		255,944
Net operating income	25,056	26,671	33,109	37,224
Other components of net periodic benefit cost	(2,463)	(2,383)	(5,009)	(4,886)
Income tax benefit (expense) on other income and expenses	819	(246)	1,577	(1,135)
Net other income	(2,204)	375	(4,099)	(830)
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		29,103		25,492			29,103	25,492
Change in restricted cash				(598)
Net cash used in investing activities				(110,076)
Change in cash, cash equivalents, and restricted cash				3,611
Cash, cash equivalents, and restricted cash at beginning of period				25,492
Cash, cash equivalents, and restricted cash at end of period		29,103		29,103
Administrative and general		23,796		49,045
Income taxes		9,635		8,751
Total operating expenses		145,873		259,859
Net operating income		25,259		33,309
Other components of net periodic benefit cost		0		0
Income tax benefit (expense) on other income and expenses		(1,217)		(2,106)
Net other income		1,787		3,085
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		1,041		443			$ 1,041	$ 443
Change in restricted cash				598
Net cash used in investing activities				598
Change in cash, cash equivalents, and restricted cash				598
Cash, cash equivalents, and restricted cash at beginning of period				443
Cash, cash equivalents, and restricted cash at end of period		1,041		1,041
Increase (Decrease) from Retrospective Adoption | Accounting Standards Update 2017-07
Summary of Significant Accounting Policies [Line Items]
Administrative and general		(2,383)		(4,886)
Income taxes		971		971
Total operating expenses		(1,412)		(3,915)
Net operating income		1,412		3,915
Other components of net periodic benefit cost		2,383		4,886
Income tax benefit (expense) on other income and expenses		971		971
Net other income		(1,412)		(3,915)
Regulated water and wastewater services
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	171,772	162,593	306,026	275,405
Total operating revenue	172,632	171,132	304,879	293,168
Operating and maintenance revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	2,297	2,053	5,462	3,966
Other non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	1,981	1,178	2,724	2,220
Non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	4,278	3,231	8,186	6,186
Total non-regulated revenue	4,845	3,739	9,264	7,201
Residential
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	109,849	102,171	201,167	178,036
Business
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	32,125	29,543	59,182	51,569
Industrial
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	7,941	6,408	15,520	13,363
Public authorities
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	8,251	7,874	13,695	12,020
Other
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 13,606	$ 16,597	$ 16,462	$ 20,417