Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Consolidating Financial Statements
Operating revenue	$ 218,983	$ 211,731	$ 523,862	$ 504,899
Operations:
Water production costs	78,818	75,261	191,797	181,460
Administrative and general	26,493	22,749	77,195	66,908
Other operations	21,943	21,208	60,307	55,660
Maintenance	6,768	6,057	17,596	16,877
Depreciation and amortization	21,009	19,231	62,677	57,650
Income tax (benefit) expense	11,262	18,219	15,380	27,941
Property and other taxes	7,142	6,544	20,253	18,717
Total operating (income) expenses	173,435	169,269	445,205	425,213
Net operating income	45,548	42,462	78,657	79,686
Other income and expenses:
Non-regulated revenue	4,703	3,542	13,967	10,743
Non-regulated expenses	(4,897)	(2,576)	(16,449)	(6,244)
Other components of net periodic benefit cost	(1,975)	(2,137)	(6,984)	(7,023)
Allowance for equity funds used during construction	1,023	1,105	2,644	2,763
Income tax benefit (expense) on other income and expenses	305	30	1,882	(1,105)
Net other loss	(841)	(36)	(4,940)	(866)
Interest expense:
Interest expense	10,875	9,284	30,207	27,073
Allowance for borrowed funds used during construction	(560)	(707)	(1,359)	(1,765)
Net interest expense	10,315	8,577	28,848	25,308
Equity earnings of subsidiaries	0	0	0	0
Net income	34,392	33,849	44,869	53,512
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)	(127)	(436)	(379)
Maintenance	0	0	0	0
Depreciation and amortization	(22)	(22)	(65)	(68)
Income tax (benefit) expense	222	266	620	786
Property and other taxes	0	0	0	0
Total operating (income) expenses	56	117	119	339
Net operating income	(56)	(117)	(119)	(339)
Other income and expenses:
Non-regulated revenue	(773)	(631)	(2,152)	(1,861)
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	216	257	602	758
Net other loss	(557)	(374)	(1,550)	(1,103)
Interest expense:
Interest expense	(628)	(505)	(1,716)	(1,482)
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	(628)	(505)	(1,716)	(1,482)
Equity earnings of subsidiaries	(34,307)	(33,748)	(44,516)	(53,161)
Net income	(34,292)	(33,734)	(44,469)	(53,121)
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	23	21	70	70
Income tax (benefit) expense	(142)	(136)	(342)	(362)
Property and other taxes	0	0	0	(4)
Total operating (income) expenses	(119)	(115)	(272)	(296)
Net operating income	119	115	272	296
Other income and expenses:
Non-regulated revenue	628	505	1,716	1,482
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	(176)	(206)	(480)	(604)
Net other loss	452	299	1,236	878
Interest expense:
Interest expense	486	313	1,155	823
Allowance for borrowed funds used during construction	0	0	0	0
Net interest expense	486	313	1,155	823
Equity earnings of subsidiaries	34,307	33,748	44,516	53,161
Net income	34,392	33,849	44,869	53,512
Cal Water | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	206,390	199,002	492,256	473,518
Operations:
Water production costs	76,317	73,061	185,149	175,339
Administrative and general	23,878	20,352	69,531	59,380
Other operations	20,271	18,979	55,626	50,108
Maintenance	6,538	5,729	16,974	16,144
Depreciation and amortization	19,632	18,115	58,909	54,328
Income tax (benefit) expense	9,911	17,009	13,511	26,075
Property and other taxes	6,205	5,680	17,894	16,407
Total operating (income) expenses	162,752	158,925	417,594	397,781
Net operating income	43,638	40,077	74,662	75,737
Other income and expenses:
Non-regulated revenue	4,589	3,218	13,572	9,822
Non-regulated expenses	(4,675)	(2,151)	(15,943)	(5,326)
Other components of net periodic benefit cost	(1,834)	(2,010)	(6,618)	(6,605)
Allowance for equity funds used during construction	1,023	1,105	2,644	2,763
Income tax benefit (expense) on other income and expenses	252	(66)	1,776	(1,227)
Net other loss	(645)	96	(4,569)	(573)
Interest expense:
Interest expense	10,443	8,951	29,095	26,216
Allowance for borrowed funds used during construction	(522)	(684)	(1,250)	(1,702)
Net interest expense	9,921	8,267	27,845	24,514
Equity earnings of subsidiaries	0	0	0	0
Net income	33,072	31,906	42,248	50,650
All Other Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	12,593	12,729	31,606	31,381
Operations:
Water production costs	2,501	2,200	6,648	6,121
Administrative and general	2,615	2,397	7,664	7,528
Other operations	1,816	2,356	5,117	5,931
Maintenance	230	328	622	733
Depreciation and amortization	1,376	1,117	3,763	3,320
Income tax (benefit) expense	1,271	1,080	1,591	1,442
Property and other taxes	937	864	2,359	2,314
Total operating (income) expenses	10,746	10,342	27,764	27,389
Net operating income	1,847	2,387	3,842	3,992
Other income and expenses:
Non-regulated revenue	259	450	831	1,300
Non-regulated expenses	(222)	(425)	(506)	(918)
Other components of net periodic benefit cost	(141)	(127)	(366)	(418)
Allowance for equity funds used during construction	0	0	0	0
Income tax benefit (expense) on other income and expenses	13	45	(16)	(32)
Net other loss	(91)	(57)	(57)	(68)
Interest expense:
Interest expense	574	525	1,673	1,516
Allowance for borrowed funds used during construction	(38)	(23)	(109)	(63)
Net interest expense	536	502	1,564	1,453
Equity earnings of subsidiaries	0	0	0	0
Net income	$ 1,220	$ 1,828	$ 2,221	$ 2,471