Condensed Consolidating Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Utility plant:
Utility plant	$ 3,411,219			$ 3,229,446
Less accumulated depreciation and amortization	(1,067,965)			(996,723)
Net utility plant	2,343,254			2,232,723
Current assets:
Cash and cash equivalents	51,257			47,176
Receivables and unbilled revenue	138,600			122,959
Receivables from affiliates	0			0
Other current assets	22,199			18,567
Total current assets	212,056			188,702
Other assets:
Regulatory assets	382,484			353,569
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	85,460			62,710
Total other assets	467,944			416,279
TOTAL ASSETS	3,023,254			2,837,704
Capitalization:
Common stockholders’ equity	757,795	$ 723,164	$ 714,129	730,157	$ 723,140	$ 695,257	$ 688,853	$ 699,221	$ 699,221
Affiliate long-term debt	0			0
Long-term debt, net	807,478			710,027
Total capitalization	1,565,273			1,440,184
Current liabilities:
Current maturities of long-term debt, net	5,280			104,911
Short-term borrowings	155,100			65,100
Payables to affiliates	0			0
Accounts payable	108,593			95,580
Accrued expenses and other liabilities	64,971			55,575
Total current liabilities	333,944			321,166
Unamortized investment tax credits	1,649			1,649
Deferred income taxes	229,237			213,033
Pension and postretirement benefits other than pensions	203,557			193,538
Regulatory liabilities and other	260,812			256,522
Advances for construction	190,272			186,342
Contributions in aid of construction	238,510			225,270
TOTAL CAPITALIZATION AND LIABILITIES	3,023,254			2,837,704
Consolidating Adjustments
Utility plant:
Utility plant	(7,197)			(7,197)
Less accumulated depreciation and amortization	2,159			2,097
Net utility plant	(5,038)			(5,100)
Current assets:
Cash and cash equivalents	0			0
Receivables and unbilled revenue	0			0
Receivables from affiliates	(29,650)			(25,453)
Other current assets	0			0
Total current assets	(29,650)			(25,453)
Other assets:
Regulatory assets	0			0
Investments in affiliates	(756,177)			(733,156)
Long-term affiliate notes receivable	(26,288)			(27,829)
Other assets	(214)			(203)
Total other assets	(782,679)			(761,188)
TOTAL ASSETS	(817,367)			(791,741)
Capitalization:
Common stockholders’ equity	(761,403)			(738,433)
Affiliate long-term debt	(26,288)			(27,828)
Long-term debt, net	0			0
Total capitalization	(787,691)			(766,261)
Current liabilities:
Current maturities of long-term debt, net	0			0
Short-term borrowings	0			0
Payables to affiliates	(29,650)			(25,453)
Accounts payable	0			0
Accrued expenses and other liabilities	0			0
Total current liabilities	(29,650)			(25,453)
Unamortized investment tax credits	0			0
Deferred income taxes	0			(43)
Pension and postretirement benefits other than pensions	(26)			0
Regulatory liabilities and other	0			16
Advances for construction	0			0
Contributions in aid of construction	0			0
TOTAL CAPITALIZATION AND LIABILITIES	(817,367)			(791,741)
Parent Company | Reportable Legal Entities
Utility plant:
Utility plant	1,318			1,318
Less accumulated depreciation and amortization	(1,083)			(1,013)
Net utility plant	235			305
Current assets:
Cash and cash equivalents	2,408			3,779
Receivables and unbilled revenue	0			126
Receivables from affiliates	28,896			21,318
Other current assets	235			80
Total current assets	31,539			25,303
Other assets:
Regulatory assets	0			0
Investments in affiliates	756,177			733,156
Long-term affiliate notes receivable	26,288			27,829
Other assets	460			133
Total other assets	782,925			761,118
TOTAL ASSETS	814,699			786,726
Capitalization:
Common stockholders’ equity	757,795			730,157
Affiliate long-term debt	0			0
Long-term debt, net	0			0
Total capitalization	757,795			730,157
Current liabilities:
Current maturities of long-term debt, net	0			0
Short-term borrowings	55,100			55,100
Payables to affiliates	0			17
Accounts payable	0			0
Accrued expenses and other liabilities	329			107
Total current liabilities	55,429			55,224
Unamortized investment tax credits	0			0
Deferred income taxes	1,475			1,376
Pension and postretirement benefits other than pensions	0			0
Regulatory liabilities and other	0			(31)
Advances for construction	0			0
Contributions in aid of construction	0			0
TOTAL CAPITALIZATION AND LIABILITIES	814,699			786,726
Cal Water | Reportable Legal Entities
Utility plant:
Utility plant	3,194,372			3,021,437
Less accumulated depreciation and amortization	(1,004,387)			(938,072)
Net utility plant	2,189,985			2,083,365
Current assets:
Cash and cash equivalents	39,187			33,763
Receivables and unbilled revenue	133,281			118,632
Receivables from affiliates	540			4,074
Other current assets	19,435			16,907
Total current assets	192,443			173,376
Other assets:
Regulatory assets	377,962			349,414
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	80,480			58,959
Total other assets	458,442			408,373
TOTAL ASSETS	2,840,870			2,665,114
Capitalization:
Common stockholders’ equity	679,037			659,340
Affiliate long-term debt	0			0
Long-term debt, net	807,019			709,444
Total capitalization	1,486,056			1,368,784
Current liabilities:
Current maturities of long-term debt, net	5,122			104,664
Short-term borrowings	100,000			10,000
Payables to affiliates	4,906			488
Accounts payable	103,748			92,310
Accrued expenses and other liabilities	60,621			53,655
Total current liabilities	274,397			261,117
Unamortized investment tax credits	1,649			1,649
Deferred income taxes	224,549			210,052
Pension and postretirement benefits other than pensions	203,557			193,538
Regulatory liabilities and other	253,843			250,720
Advances for construction	189,781			185,843
Contributions in aid of construction	207,038			193,411
TOTAL CAPITALIZATION AND LIABILITIES	2,840,870			2,665,114
All Other Subsidiaries | Reportable Legal Entities
Utility plant:
Utility plant	222,726			213,888
Less accumulated depreciation and amortization	(64,654)			(59,735)
Net utility plant	158,072			154,153
Current assets:
Cash and cash equivalents	9,662			9,634
Receivables and unbilled revenue	5,319			4,201
Receivables from affiliates	214			61
Other current assets	2,529			1,580
Total current assets	17,724			15,476
Other assets:
Regulatory assets	4,522			4,155
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	4,692			3,821
Total other assets	9,214			7,976
TOTAL ASSETS	185,010			177,605
Capitalization:
Common stockholders’ equity	82,366			79,093
Affiliate long-term debt	26,288			27,828
Long-term debt, net	459			583
Total capitalization	109,113			107,504
Current liabilities:
Current maturities of long-term debt, net	158			247
Short-term borrowings	0			0
Payables to affiliates	24,744			24,948
Accounts payable	4,845			3,270
Accrued expenses and other liabilities	4,021			1,813
Total current liabilities	33,768			30,278
Unamortized investment tax credits	0			0
Deferred income taxes	3,213			1,648
Pension and postretirement benefits other than pensions	0			0
Regulatory liabilities and other	6,953			5,817
Advances for construction	491			499
Contributions in aid of construction	31,472			31,859
TOTAL CAPITALIZATION AND LIABILITIES	$ 185,010			$ 177,605