Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Consolidating Financial Statements
Operating revenue	$ 179,031		$ 174,938		$ 305,142	$ 309,491
Operations:
Water production costs	64,635		65,373		110,227	112,979
Administrative and general	25,434		24,383		54,531	50,702
Other operations	22,542		20,724		40,363	38,364
Maintenance	5,692		5,389		12,147	10,828
Depreciation and amortization	22,326		20,953		44,694	41,668
Income tax (benefit) expense	4,321		4,870		1,330	5,164
Property and other taxes	7,068		6,407		14,361	13,111
Total operating (income) expenses	152,018		148,099		277,653	272,816
Net operating income	27,013		26,839		27,489	36,675
Other income and expenses:
Non-regulated revenue	5,130		4,845		10,031	9,264
Non-regulated expenses	(3,900)		(6,115)		(6,119)	(11,552)
Other components of net periodic benefit cost	(1,192)		(2,463)		(2,451)	(5,009)
Allowance for equity funds used during construction	1,686		710		3,219	1,621
Income tax (expense) benefit on other income and expenses	(487)		819		(1,315)	1,577
Net other income (loss)	1,237		(2,204)		3,365	(4,099)
Interest expense:
Interest expense	12,178		10,134		23,253	19,332
Allowance for borrowed funds used during construction	(924)		(304)		(1,755)	(799)
Net interest expense	11,254		9,830		21,498	18,533
Equity earnings of subsidiaries	0		0		0	0
Net income (loss)	16,996	$ (7,640)	14,805	$ (762)	9,356	14,043
Consolidating Adjustments
Condensed Consolidating Financial Statements
Operating revenue	0		0		0	0
Operations:
Water production costs	0		0		0	0
Administrative and general	0		0		0	0
Other operations	(144)		(146)		(290)	(292)
Maintenance	0		0		0	0
Depreciation and amortization	(20)		(21)		(41)	(43)
Income tax (benefit) expense	218		201		436	398
Property and other taxes	0		0		0	0
Total operating (income) expenses	54		34		105	63
Net operating income	(54)		(34)		(105)	(63)
Other income and expenses:
Non-regulated revenue	(759)		(703)		(1,518)	(1,379)
Non-regulated expenses	0		0		0	0
Other components of net periodic benefit cost	0		0		0	0
Allowance for equity funds used during construction	0		0		0	0
Income tax (expense) benefit on other income and expenses	213		197		425	386
Net other income (loss)	(546)		(506)		(1,093)	(993)
Interest expense:
Interest expense	(614)		(557)		(1,227)	(1,088)
Allowance for borrowed funds used during construction	0		0		0	0
Net interest expense	(614)		(557)		(1,227)	(1,088)
Equity earnings of subsidiaries	(16,923)		(14,717)		(9,189)	(13,775)
Net income (loss)	(16,909)		(14,700)		(9,160)	(13,743)
Parent Company | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	0		0		0	0
Operations:
Water production costs	0		0		0	0
Administrative and general	23		0		23	0
Other operations	0		0		0	0
Maintenance	0		0		0	0
Depreciation and amortization	24		24		47	47
Income tax (benefit) expense	(144)		(122)		(279)	(200)
Property and other taxes	0		0		0	0
Total operating (income) expenses	(97)		(98)		(209)	(153)
Net operating income	97		98		209	153
Other income and expenses:
Non-regulated revenue	614		557		1,227	1,088
Non-regulated expenses	0		0		0	0
Other components of net periodic benefit cost	0		0		0	0
Allowance for equity funds used during construction	0		0		0	0
Income tax (expense) benefit on other income and expenses	(171)		(156)		(343)	(304)
Net other income (loss)	443		401		884	784
Interest expense:
Interest expense	467		411		926	669
Allowance for borrowed funds used during construction	0		0		0	0
Net interest expense	467		411		926	669
Equity earnings of subsidiaries	16,923		14,717		9,189	13,775
Net income (loss)	16,996		14,805		9,356	14,043
Cal Water | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	167,450		164,602		283,524	290,478
Operations:
Water production costs	62,143		63,209		105,569	108,832
Administrative and general	23,207		22,047		49,410	45,653
Other operations	20,857		19,138		36,979	35,355
Maintenance	5,423		5,192		11,646	10,436
Depreciation and amortization	20,814		19,664		41,701	39,277
Income tax (benefit) expense	3,792		4,463		687	4,646
Property and other taxes	6,305		5,682		12,811	11,689
Total operating (income) expenses	142,541		139,395		258,803	255,888
Net operating income	24,909		25,207		24,721	34,590
Other income and expenses:
Non-regulated revenue	4,881		4,739		9,509	8,983
Non-regulated expenses	(3,665)		(5,975)		(5,703)	(11,268)
Other components of net periodic benefit cost	(1,164)		(2,337)		(2,393)	(4,784)
Allowance for equity funds used during construction	1,686		710		3,219	1,621
Income tax (expense) benefit on other income and expenses	(486)		783		(1,296)	1,524
Net other income (loss)	1,252		(2,080)		3,336	(3,924)
Interest expense:
Interest expense	11,708		9,718		22,321	18,652
Allowance for borrowed funds used during construction	(862)		(270)		(1,638)	(728)
Net interest expense	10,846		9,448		20,683	17,924
Equity earnings of subsidiaries	0		0		0	0
Net income (loss)	15,315		13,679		7,374	12,742
All Other Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	11,581		10,336		21,618	19,013
Operations:
Water production costs	2,492		2,164		4,658	4,147
Administrative and general	2,204		2,336		5,098	5,049
Other operations	1,829		1,732		3,674	3,301
Maintenance	269		197		501	392
Depreciation and amortization	1,508		1,286		2,987	2,387
Income tax (benefit) expense	455		328		486	320
Property and other taxes	763		725		1,550	1,422
Total operating (income) expenses	9,520		8,768		18,954	17,018
Net operating income	2,061		1,568		2,664	1,995
Other income and expenses:
Non-regulated revenue	394		252		813	572
Non-regulated expenses	(235)		(140)		(416)	(284)
Other components of net periodic benefit cost	(28)		(126)		(58)	(225)
Allowance for equity funds used during construction	0		0		0	0
Income tax (expense) benefit on other income and expenses	(43)		(5)		(101)	(29)
Net other income (loss)	88		(19)		238	34
Interest expense:
Interest expense	617		562		1,233	1,099
Allowance for borrowed funds used during construction	(62)		(34)		(117)	(71)
Net interest expense	555		528		1,116	1,028
Equity earnings of subsidiaries	0		0		0	0
Net income (loss)	$ 1,594		$ 1,021		$ 1,786	$ 1,001