Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Condensed Consolidating Financial Statements
Operating revenue	$ 211,731	$ 184,268	$ 504,899	$ 458,440
Operations:
Water production costs	75,261	70,175	181,460	168,833
Administrative and general	24,886	23,844	73,931	75,037
Other operations	21,208	19,561	55,660	57,766
Maintenance	6,057	5,545	16,877	17,542
Depreciation and amortization	19,231	15,884	57,650	47,772
Income tax (benefit) expense	17,348	13,247	26,099	19,192
Property and other taxes	6,544	5,957	18,717	17,439
Total operating (income) expense	170,535	154,213	430,394	403,581
Net operating income (loss)	41,196	30,055	74,505	54,859
Other income and expenses:
Non-regulated revenue	3,542	3,397	10,743	10,589
Non-regulated expenses	(2,576)	(2,517)	(6,244)	(8,306)
Allowance for equity funds used during construction	1,105	0	2,763	0
Income tax expense on other income and expenses	(841)	(349)	(2,947)	(914)
Net other income	1,230	531	4,315	1,369
Interest expense:
Interest expense	9,284	8,485	27,073	24,984
Allowance for borrowed funds used during construction	(707)	(774)	(1,765)	(2,341)
Net interest expense	8,577	7,711	25,308	22,643
Equity earnings of subsidiaries	0	0	0	0
Net income	33,849	22,875	53,512	33,585
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	(127)	(168)	(379)	(378)
Maintenance	0	0	0	0
Depreciation and amortization	(22)	(24)	(68)	(72)
Income tax (benefit) expense	266	267	786	762
Property and other taxes	0	0	0	0
Total operating (income) expense	117	75	339	312
Net operating income (loss)	(117)	(75)	(339)	(312)
Other income and expenses:
Non-regulated revenue	(631)	(632)	(1,861)	(1,798)
Non-regulated expenses	0	0	0	0
Allowance for equity funds used during construction	0		0
Income tax expense on other income and expenses	257	258	758	733
Net other income	(374)	(374)	(1,103)	(1,065)
Interest expense:
Interest expense	(505)	(463)	(1,482)	(1,419)
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	(505)	(463)	(1,482)	(1,419)
Equity earnings of subsidiaries	(33,748)	(22,753)	(53,161)	(33,187)
Net income	(33,734)	(22,739)	(53,121)	(33,145)
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	21	57	70	171
Income tax (benefit) expense	(136)	(105)	(362)	(292)
Property and other taxes	0	0	(4)	0
Total operating (income) expense	(115)	(48)	(296)	(121)
Net operating income (loss)	115	48	296	121
Other income and expenses:
Non-regulated revenue	505	464	1,482	1,390
Non-regulated expenses	0	0	0	0
Allowance for equity funds used during construction	0		0
Income tax expense on other income and expenses	(206)	(189)	(604)	(566)
Net other income	299	275	878	824
Interest expense:
Interest expense	313	201	823	547
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	313	201	823	547
Equity earnings of subsidiaries	33,748	22,753	53,161	33,187
Net income	33,849	22,875	53,512	33,585
Cal Water | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	199,002	173,223	473,518	428,592
Operations:
Water production costs	73,061	68,045	175,339	162,933
Administrative and general	22,362	21,679	65,985	67,289
Other operations	18,979	18,037	50,108	53,128
Maintenance	5,729	5,322	16,144	16,854
Depreciation and amortization	18,115	14,777	54,328	44,427
Income tax (benefit) expense	16,190	12,165	24,344	17,356
Property and other taxes	5,680	5,182	16,407	15,241
Total operating (income) expense	160,116	145,207	402,655	377,228
Net operating income (loss)	38,886	28,016	70,863	51,364
Other income and expenses:
Non-regulated revenue	3,218	3,024	9,822	9,659
Non-regulated expenses	(2,151)	(2,170)	(5,326)	(7,422)
Allowance for equity funds used during construction	1,105		2,763
Income tax expense on other income and expenses	(885)	(345)	(2,958)	(909)
Net other income	1,287	509	4,301	1,328
Interest expense:
Interest expense	8,951	8,259	26,216	24,421
Allowance for borrowed funds used during construction	(684)	(759)	(1,702)	(2,293)
Net interest expense	8,267	7,500	24,514	22,128
Equity earnings of subsidiaries	0	0	0	0
Net income	31,906	21,025	50,650	30,564
All Other Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	12,729	11,045	31,381	29,848
Operations:
Water production costs	2,200	2,130	6,121	5,900
Administrative and general	2,524	2,165	7,946	7,748
Other operations	2,356	1,692	5,931	5,016
Maintenance	328	223	733	688
Depreciation and amortization	1,117	1,074	3,320	3,246
Income tax (benefit) expense	1,028	920	1,331	1,366
Property and other taxes	864	775	2,314	2,198
Total operating (income) expense	10,417	8,979	27,696	26,162
Net operating income (loss)	2,312	2,066	3,685	3,686
Other income and expenses:
Non-regulated revenue	450	541	1,300	1,338
Non-regulated expenses	(425)	(347)	(918)	(884)
Allowance for equity funds used during construction	0		0
Income tax expense on other income and expenses	(7)	(73)	(143)	(172)
Net other income	18	121	239	282
Interest expense:
Interest expense	525	488	1,516	1,435
Allowance for borrowed funds used during construction	(23)	(15)	(63)	(48)
Net interest expense	502	473	1,453	1,387
Equity earnings of subsidiaries	0	0	0	0
Net income	$ 1,828	$ 1,714	$ 2,471	$ 2,581