Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Consolidating Financial Statements
Operating revenue	$ 232,537			$ 221,288			$ 537,679	$ 530,779
Operations:
Water production costs	80,568			78,818			190,795	191,797
Administrative and general	26,779			26,493			81,310	77,195
Other operations	24,550			21,943			64,913	60,307
Maintenance	7,065			6,768			19,212	17,596
Depreciation and amortization	22,273			21,009			66,967	62,677
Income tax (benefit) expense	12,194			11,786			13,524	16,950
Property and other taxes	7,541			7,142			21,902	20,253
Total operating (income) expenses	180,970			173,959			458,623	446,775
Net operating income	51,567			47,329			79,056	84,004
Other income and expenses:
Non-regulated revenue	4,118			4,703			14,149	13,967
Non-regulated expenses	(4,351)			(4,897)			(10,470)	(16,449)
Other components of net periodic benefit cost	(1,857)			(1,975)			(4,308)	(6,984)
Allowance for equity funds used during construction	1,868			1,023			5,087	2,644
Income tax benefit on other income and expenses	330			305			(985)	1,882
Net other income (loss)	108			(841)			3,473	(4,940)
Interest expense:
Interest expense	10,279			10,875			33,532	30,207
Allowance for borrowed funds used during construction	(1,028)			(560)			(2,783)	(1,359)
Net interest expense	9,251			10,315			30,749	28,848
Equity earnings of subsidiaries	0			0			0	0
Net income	42,424	$ 16,996	$ (7,640)	36,173	$ 14,805	$ (762)	51,780	50,216
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	(147)			(144)			(437)	(436)
Maintenance	0			0			0	0
Depreciation and amortization	(21)			(22)			(62)	(65)
Income tax (benefit) expense	215			222			651	620
Property and other taxes	0			0			0	0
Total operating (income) expenses	47			56			152	119
Net operating income	(47)			(56)			(152)	(119)
Other income and expenses:
Non-regulated revenue	(745)			(773)			(2,263)	(2,152)
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 on other income and expenses	208			216			633	602
Net other income (loss)	(537)			(557)			(1,630)	(1,550)
Interest expense:
Interest expense	(599)			(628)			(1,826)	(1,716)
Allowance for borrowed funds used during construction	0			0			0	0
Net interest expense	(599)			(628)			(1,826)	(1,716)
Equity earnings of subsidiaries	(42,334)			(36,088)			(51,523)	(49,863)
Net income	(42,319)			(36,073)			(51,479)	(49,816)
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			23	0
Other operations	0			0			0	0
Maintenance	0			0			0	0
Depreciation and amortization	23			23			70	70
Income tax (benefit) expense	(132)			(142)			(411)	(342)
Property and other taxes	0			0			0	0
Total operating (income) expenses	(109)			(119)			(318)	(272)
Net operating income	109			119			318	272
Other income and expenses:
Non-regulated revenue	599			628			1,826	1,716
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 on other income and expenses	(168)			(176)			(511)	(480)
Net other income (loss)	431			452			1,315	1,236
Interest expense:
Interest expense	450			486			1,376	1,155
Allowance for borrowed funds used during construction	0			0			0	0
Net interest expense	450			486			1,376	1,155
Equity earnings of subsidiaries	42,334			36,088			51,523	49,863
Net income	42,424			36,173			51,780	50,216
Cal Water | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	219,261			208,695			502,785	499,173
Operations:
Water production costs	78,048			76,317			183,617	185,149
Administrative and general	24,498			23,878			73,908	69,531
Other operations	22,872			20,271			59,851	55,626
Maintenance	6,823			6,538			18,469	16,974
Depreciation and amortization	20,770			19,632			62,471	58,909
Income tax (benefit) expense	11,332			10,435			12,019	15,081
Property and other taxes	6,620			6,205			19,431	17,894
Total operating (income) expenses	170,963			163,276			429,766	419,164
Net operating income	48,298			45,419			73,019	80,009
Other income and expenses:
Non-regulated revenue	3,865			4,589			13,374	13,572
Non-regulated expenses	(3,907)			(4,675)			(9,610)	(15,943)
Other components of net periodic benefit cost	(1,784)			(1,834)			(4,177)	(6,618)
Allowance for equity funds used during construction	1,868			1,023			5,087	2,644
Income tax benefit on other income and expenses	268			252			(1,028)	1,776
Net other income (loss)	310			(645)			3,646	(4,569)
Interest expense:
Interest expense	9,820			10,443			32,141	29,095
Allowance for borrowed funds used during construction	(954)			(522)			(2,592)	(1,250)
Net interest expense	8,866			9,921			29,549	27,845
Equity earnings of subsidiaries	0			0			0	0
Net income	39,742			34,853			47,116	47,595
All Other Subsidiaries | Reportable Legal Entities
Condensed Consolidating Financial Statements
Operating revenue	13,276			12,593			34,894	31,606
Operations:
Water production costs	2,520			2,501			7,178	6,648
Administrative and general	2,281			2,615			7,379	7,664
Other operations	1,825			1,816			5,499	5,117
Maintenance	242			230			743	622
Depreciation and amortization	1,501			1,376			4,488	3,763
Income tax (benefit) expense	779			1,271			1,265	1,591
Property and other taxes	921			937			2,471	2,359
Total operating (income) expenses	10,069			10,746			29,023	27,764
Net operating income	3,207			1,847			5,871	3,842
Other income and expenses:
Non-regulated revenue	399			259			1,212	831
Non-regulated expenses	(444)			(222)			(860)	(506)
Other components of net periodic benefit cost	(73)			(141)			(131)	(366)
Allowance for equity funds used during construction	0			0			0	0
Income tax benefit on other income and expenses	22			13			(79)	(16)
Net other income (loss)	(96)			(91)			142	(57)
Interest expense:
Interest expense	608			574			1,841	1,673
Allowance for borrowed funds used during construction	(74)			(38)			(191)	(109)
Net interest expense	534			536			1,650	1,564
Equity earnings of subsidiaries	0			0			0	0
Net income	$ 2,577			$ 1,220			$ 4,363	$ 2,221