Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Consolidating Financial Statements
Operating revenue	$ 172,632	$ 171,132	$ 304,879	$ 293,168
Operations:
Water production costs	65,373	64,131	112,979	106,199
Administrative and general	24,383	21,413	50,702	44,159
Other operations	20,724	18,328	38,364	34,452
Maintenance	5,389	4,708	10,828	10,820
Depreciation and amortization	20,953	19,218	41,668	38,419
Income tax (benefit) expense	4,347	10,606	4,118	9,722
Property and other taxes	6,407	6,057	13,111	12,173
Total operating (income) expenses	147,576	144,461	271,770	255,944
Net operating income	25,056	26,671	33,109	37,224
Other income and expenses:
Non-regulated revenue	4,845	3,739	9,264	7,201
Non-regulated expenses	(6,115)	(1,614)	(11,552)	(3,668)
Other components of net periodic benefit cost	(2,463)	(2,383)	(5,009)	(4,886)
Allowance for equity funds used during construction	710	879	1,621	1,658
Income tax benefit (expense) on other income and expenses	819	(246)	1,577	(1,135)
Net other (loss) income	(2,204)	375	(4,099)	(830)
Interest expense:
Interest expense	10,134	9,079	19,332	17,789
Allowance for borrowed funds used during construction	(304)	(564)	(799)	(1,058)
Net interest expense	9,830	8,515	18,533	16,731
Equity earnings of subsidiaries	0	0	0	0
Net income	13,022	18,531	10,477	19,663
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	(146)	(126)	(292)	(252)
Maintenance	0	0	0	0
Depreciation and amortization	(21)	(23)	(43)	(46)
Income tax (benefit) expense	201	263	398	520
Property and other taxes	0	0	0	0
Total operating (income) expenses	34	114	63	222
Net operating income	(34)	(114)	(63)	(222)
Other income and expenses:
Non-regulated revenue	(703)	(622)	(1,379)	(1,230)
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 benefit (expense) on other income and expenses	197	253	386	501
Net other (loss) income	(506)	(369)	(993)	(729)
Interest expense:
Interest expense	(557)	(496)	(1,088)	(977)
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	(557)	(496)	(1,088)	(977)
Equity earnings of subsidiaries	(12,934)	(18,415)	(10,209)	(19,413)
Net income	(12,917)	(18,402)	(10,177)	(19,387)
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	0	0	0	0
Other operations	0	0	0	0
Maintenance	0	0	0	0
Depreciation and amortization	24	26	47	49
Income tax (benefit) expense	(122)	(123)	(200)	(226)
Property and other taxes	0	0	0	(4)
Total operating (income) expenses	(98)	(97)	(153)	(181)
Net operating income	98	97	153	181
Other income and expenses:
Non-regulated revenue	557	496	1,088	977
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 benefit (expense) on other income and expenses	(156)	(202)	(304)	(398)
Net other (loss) income	401	294	784	579
Interest expense:
Interest expense	411	275	669	510
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	411	275	669	510
Equity earnings of subsidiaries	12,934	18,415	10,209	19,413
Net income	13,022	18,531	10,477	19,663
Cal Water | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	162,296	161,174	285,866	274,516
Operations:
Water production costs	63,209	62,089	108,832	102,278
Administrative and general	22,047	18,907	45,653	39,028
Other operations	19,138	16,729	35,355	31,129
Maintenance	5,192	4,509	10,436	10,415
Depreciation and amortization	19,664	18,102	39,277	36,213
Income tax (benefit) expense	3,940	10,012	3,600	9,066
Property and other taxes	5,682	5,315	11,689	10,727
Total operating (income) expenses	138,872	135,663	254,842	238,856
Net operating income	23,424	25,511	31,024	35,660
Other income and expenses:
Non-regulated revenue	4,739	3,469	8,983	6,604
Non-regulated expenses	(5,975)	(1,428)	(11,268)	(3,175)
Other components of net periodic benefit cost	(2,337)	(2,240)	(4,784)	(4,595)
Allowance for equity funds used during construction	710	879	1,621	1,658
Income tax benefit (expense) on other income and expenses	783	(278)	1,524	(1,161)
Net other (loss) income	(2,080)	402	(3,924)	(669)
Interest expense:
Interest expense	9,718	8,795	18,652	17,265
Allowance for borrowed funds used during construction	(270)	(542)	(728)	(1,018)
Net interest expense	9,448	8,253	17,924	16,247
Equity earnings of subsidiaries	0	0	0	0
Net income	11,896	17,660	9,176	18,744
All Other Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	10,336	9,958	19,013	18,652
Operations:
Water production costs	2,164	2,042	4,147	3,921
Administrative and general	2,336	2,506	5,049	5,131
Other operations	1,732	1,725	3,301	3,575
Maintenance	197	199	392	405
Depreciation and amortization	1,286	1,113	2,387	2,203
Income tax (benefit) expense	328	454	320	362
Property and other taxes	725	742	1,422	1,450
Total operating (income) expenses	8,768	8,781	17,018	17,047
Net operating income	1,568	1,177	1,995	1,605
Other income and expenses:
Non-regulated revenue	252	396	572	850
Non-regulated expenses	(140)	(186)	(284)	(493)
Other components of net periodic benefit cost	(126)	(143)	(225)	(291)
Allowance for equity funds used during construction	0	0	0	0
Income tax benefit (expense) on other income and expenses	(5)	(19)	(29)	(77)
Net other (loss) income	(19)	48	34	(11)
Interest expense:
Interest expense	562	505	1,099	991
Allowance for borrowed funds used during construction	(34)	(22)	(71)	(40)
Net interest expense	528	483	1,028	951
Equity earnings of subsidiaries	0	0	0	0
Net income	$ 1,021	$ 742	$ 1,001	$ 643