Condensed Consolidating Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Utility plant:
Utility plant	$ 2,892,666	$ 2,717,339
Less accumulated depreciation and amortization	(910,742)	(858,062)
Net utility plant	1,981,924	1,859,277
Current assets:
Cash and cash equivalents	28,341	25,492	$ 21,351	$ 8,837
Receivables and unbilled revenue, net	133,256
Receivables		103,023
Receivables from affiliates	0	0
Other current assets	18,888	13,554
Total current assets	180,485	142,069
Other assets:
Regulatory assets	379,884	355,930
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	60,811	54,469
Total other assets	440,695	410,399
TOTAL ASSETS	2,603,104	2,411,745
Capitalization:
Common stockholders' equity	687,723	659,471
Affiliate long-term debt	0	0
Long-term debt, less current maturities	519,700	531,745
Total capitalization	1,207,423	1,191,216
Current liabilities:
Current maturities of long-term debt	36,015	26,208
Short-term borrowings	195,100	97,100
Payables to affiliates	0	0
Accounts payable	89,394	77,813
Accrued expenses and other liabilities	59,852	49,109
Total current liabilities	380,361	250,230
Unamortized investment tax credits	1,798	1,798
Deferred income taxes	329,506	298,924
Pension and postretirement benefits other than pensions	227,819	222,691
Regulatory liabilities and other	91,006	83,648
Advances for construction	182,820	182,448
Contributions in aid of construction	182,371	180,790
TOTAL CAPITALIZATION AND LIABILITIES	2,603,104	2,411,745
Consolidating Adjustments
Utility plant:
Utility plant	(7,196)	(7,197)
Less accumulated depreciation and amortization	1,988	1,919
Net utility plant	(5,208)	(5,278)
Current assets:
Cash and cash equivalents	0	0	0	0
Receivables and unbilled revenue, net	0
Receivables		0
Receivables from affiliates	(23,532)	(23,182)
Other current assets	0	0
Total current assets	(23,532)	(23,182)
Other assets:
Regulatory assets	0	0
Investments in affiliates	(693,766)	(666,525)
Long-term affiliate notes receivable	(24,677)	(25,744)
Other assets	(204)	(33)
Total other assets	(718,647)	(692,302)
TOTAL ASSETS	(747,387)	(720,762)
Capitalization:
Common stockholders' equity	(699,035)	(671,836)
Affiliate long-term debt	(24,677)	(25,744)
Long-term debt, less current maturities	0	0
Total capitalization	(723,712)	(697,580)
Current liabilities:
Current maturities of long-term debt	0	0
Short-term borrowings	0	0
Payables to affiliates	(23,532)	(23,182)
Accounts payable	0	0
Accrued expenses and other liabilities	0	0
Total current liabilities	(23,532)	(23,182)
Unamortized investment tax credits	0	0
Deferred income taxes	(143)	0
Pension and postretirement benefits other than pensions	0	0
Regulatory liabilities and other	0	0
Advances for construction	0	0
Contributions in aid of construction	0	0
TOTAL CAPITALIZATION AND LIABILITIES	(747,387)	(720,762)
Parent Company | Reportable Legal Entities
Utility plant:
Utility plant	1,321	1,318
Less accumulated depreciation and amortization	(896)	(826)
Net utility plant	425	492
Current assets:
Cash and cash equivalents	2,722	5,216	1,987	582
Receivables and unbilled revenue, net	0
Receivables		0
Receivables from affiliates	22,568	19,566
Other current assets	184	80
Total current assets	25,474	24,862
Other assets:
Regulatory assets	0	0
Investments in affiliates	693,766	666,525
Long-term affiliate notes receivable	24,677	25,744
Other assets	217	376
Total other assets	718,660	692,645
TOTAL ASSETS	744,559	717,999
Capitalization:
Common stockholders' equity	687,723	659,471
Affiliate long-term debt	0	0
Long-term debt, less current maturities	0	0
Total capitalization	687,723	659,471
Current liabilities:
Current maturities of long-term debt	0	0
Short-term borrowings	55,100	57,100
Payables to affiliates	0	0
Accounts payable	0	0
Accrued expenses and other liabilities	154	88
Total current liabilities	55,254	57,188
Unamortized investment tax credits	0	0
Deferred income taxes	1,582	1,340
Pension and postretirement benefits other than pensions	0	0
Regulatory liabilities and other	0	0
Advances for construction	0	0
Contributions in aid of construction	0	0
TOTAL CAPITALIZATION AND LIABILITIES	744,559	717,999
Cal Water | Reportable Legal Entities
Utility plant:
Utility plant	2,695,106	2,519,785
Less accumulated depreciation and amortization	(858,386)	(805,992)
Net utility plant	1,836,720	1,713,793
Current assets:
Cash and cash equivalents	17,851	13,215	13,928	4,270
Receivables and unbilled revenue, net	127,858
Receivables		98,850
Receivables from affiliates	703	3,608
Other current assets	17,665	12,442
Total current assets	164,077	128,115
Other assets:
Regulatory assets	376,041	352,139
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	56,886	50,361
Total other assets	432,927	402,500
TOTAL ASSETS	2,433,724	2,244,408
Capitalization:
Common stockholders' equity	620,892	595,003
Affiliate long-term debt	0	0
Long-term debt, less current maturities	518,802	530,850
Total capitalization	1,139,694	1,125,853
Current liabilities:
Current maturities of long-term debt	35,606	25,657
Short-term borrowings	140,000	40,000
Payables to affiliates	1,257	539
Accounts payable	86,374	74,998
Accrued expenses and other liabilities	55,607	47,232
Total current liabilities	318,844	188,426
Unamortized investment tax credits	1,798	1,798
Deferred income taxes	325,619	296,781
Pension and postretirement benefits other than pensions	227,819	222,691
Regulatory liabilities and other	87,714	80,518
Advances for construction	182,298	181,907
Contributions in aid of construction	149,938	146,434
TOTAL CAPITALIZATION AND LIABILITIES	2,433,724	2,244,408
All Other Subsidiaries | Reportable Legal Entities
Utility plant:
Utility plant	203,435	203,433
Less accumulated depreciation and amortization	(53,448)	(53,163)
Net utility plant	149,987	150,270
Current assets:
Cash and cash equivalents	7,768	7,061	$ 5,436	$ 3,985
Receivables and unbilled revenue, net	5,398
Receivables		4,173
Receivables from affiliates	261	8
Other current assets	1,039	1,032
Total current assets	14,466	12,274
Other assets:
Regulatory assets	3,843	3,791
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	3,912	3,765
Total other assets	7,755	7,556
TOTAL ASSETS	172,208	170,100
Capitalization:
Common stockholders' equity	78,143	76,833
Affiliate long-term debt	24,677	25,744
Long-term debt, less current maturities	898	895
Total capitalization	103,718	103,472
Current liabilities:
Current maturities of long-term debt	409	551
Short-term borrowings	0	0
Payables to affiliates	22,275	22,643
Accounts payable	3,020	2,815
Accrued expenses and other liabilities	4,091	1,789
Total current liabilities	29,795	27,798
Unamortized investment tax credits	0	0
Deferred income taxes	2,448	803
Pension and postretirement benefits other than pensions	0	0
Regulatory liabilities and other	3,292	3,130
Advances for construction	522	541
Contributions in aid of construction	32,433	34,356
TOTAL CAPITALIZATION AND LIABILITIES	$ 172,208	$ 170,100