Condensed Consolidating Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Utility plant:
Utility plant	$ 2,826,187	$ 2,717,339
Less accumulated depreciation and amortization	(897,575)	(858,062)
Net utility plant	1,928,612	1,859,277
Current assets:
Cash and cash equivalents	29,103	25,492	$ 30,826	$ 8,837
Receivables and unbilled revenue, net	120,277
Receivables		103,023
Receivables from affiliates	0	0
Other current assets	21,200	13,554
Total current assets	170,580	142,069
Other assets:
Regulatory assets	375,868	355,930
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	57,966	54,469
Total other assets	433,834	410,399
TOTAL ASSETS	2,533,026	2,411,745
Capitalization:
Common stockholders' equity	661,834	659,471
Affiliate long-term debt	0	0
Long-term debt, less current maturities	519,875	531,745
Total capitalization	1,181,709	1,191,216
Current liabilities:
Current maturities of long-term debt	36,110	26,208
Short-term borrowings	190,100	97,100
Payables to affiliates	0	0
Accounts payable	84,222	77,813
Accrued expenses and other liabilities	50,595	49,109
Total current liabilities	361,027	250,230
Unamortized investment tax credits	1,798	1,798
Deferred income taxes	310,936	298,924
Pension and postretirement benefits other than pensions	227,186	222,691
Regulatory liabilities and other	86,821	83,648
Advances for construction	182,394	182,448
Contributions in aid of construction	181,155	180,790
TOTAL CAPITALIZATION AND LIABILITIES	2,533,026	2,411,745
Consolidating Adjustments
Utility plant:
Utility plant	(7,197)	(7,197)
Less accumulated depreciation and amortization	1,965	1,919
Net utility plant	(5,232)	(5,278)
Current assets:
Cash and cash equivalents	0	0	0	0
Receivables and unbilled revenue, net	0
Receivables		0
Receivables from affiliates	(23,046)	(23,182)
Other current assets	0	0
Total current assets	(23,046)	(23,182)
Other assets:
Regulatory assets	0	0
Investments in affiliates	(668,660)	(666,525)
Long-term affiliate notes receivable	(25,037)	(25,744)
Other assets	(205)	(33)
Total other assets	(693,902)	(692,302)
TOTAL ASSETS	(722,180)	(720,762)
Capitalization:
Common stockholders' equity	(673,944)	(671,836)
Affiliate long-term debt	(25,037)	(25,744)
Long-term debt, less current maturities	0	0
Total capitalization	(698,981)	(697,580)
Current liabilities:
Current maturities of long-term debt	0	0
Short-term borrowings	0	0
Payables to affiliates	(23,046)	(23,182)
Accounts payable	0	0
Accrued expenses and other liabilities	0	0
Total current liabilities	(23,046)	(23,182)
Unamortized investment tax credits	0	0
Deferred income taxes	(153)	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	(722,180)	(720,762)
Parent Company
Utility plant:
Utility plant	1,321	1,318
Less accumulated depreciation and amortization	(872)	(826)
Net utility plant	449	492
Current assets:
Cash and cash equivalents	2,112	5,216	999	582
Receivables and unbilled revenue, net	0
Receivables		0
Receivables from affiliates	21,786	19,566
Other current assets	290	80
Total current assets	24,188	24,862
Other assets:
Regulatory assets	0	0
Investments in affiliates	668,660	666,525
Long-term affiliate notes receivable	25,037	25,744
Other assets	238	376
Total other assets	693,935	692,645
TOTAL ASSETS	718,572	717,999
Capitalization:
Common stockholders' equity	661,834	659,471
Affiliate long-term debt	0	0
Long-term debt, less current maturities	0	0
Total capitalization	661,834	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	126	88
Total current liabilities	55,226	57,188
Unamortized investment tax credits	0	0
Deferred income taxes	1,512	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	718,572	717,999
Cal Water
Utility plant:
Utility plant	2,625,435	2,519,785
Less accumulated depreciation and amortization	(842,974)	(805,992)
Net utility plant	1,782,461	1,713,793
Current assets:
Cash and cash equivalents	20,417	13,215	24,606	4,270
Receivables and unbilled revenue, net	115,606
Receivables		98,850
Receivables from affiliates	1,117	3,608
Other current assets	19,810	12,442
Total current assets	156,950	128,115
Other assets:
Regulatory assets	372,048	352,139
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	53,928	50,361
Total other assets	425,976	402,500
TOTAL ASSETS	2,365,387	2,244,408
Capitalization:
Common stockholders' equity	597,262	595,003
Affiliate long-term debt	0	0
Long-term debt, less current maturities	518,839	530,850
Total capitalization	1,116,101	1,125,853
Current liabilities:
Current maturities of long-term debt	35,624	25,657
Short-term borrowings	135,000	40,000
Payables to affiliates	302	539
Accounts payable	81,399	74,998
Accrued expenses and other liabilities	47,124	47,232
Total current liabilities	299,449	188,426
Unamortized investment tax credits	1,798	1,798
Deferred income taxes	308,164	296,781
Pension and postretirement benefits other than pensions	227,186	222,691
Regulatory liabilities and other	83,579	80,518
Advances for construction	181,832	181,907
Contributions in aid of construction	147,278	146,434
TOTAL CAPITALIZATION AND LIABILITIES	2,365,387	2,244,408
All Other Subsidiaries
Utility plant:
Utility plant	206,628	203,433
Less accumulated depreciation and amortization	(55,694)	(53,163)
Net utility plant	150,934	150,270
Current assets:
Cash and cash equivalents	6,574	7,061	$ 5,221	$ 3,985
Receivables and unbilled revenue, net	4,671
Receivables		4,173
Receivables from affiliates	143	8
Other current assets	1,100	1,032
Total current assets	12,488	12,274
Other assets:
Regulatory assets	3,820	3,791
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	4,005	3,765
Total other assets	7,825	7,556
TOTAL ASSETS	171,247	170,100
Capitalization:
Common stockholders' equity	76,682	76,833
Affiliate long-term debt	25,037	25,744
Long-term debt, less current maturities	1,036	895
Total capitalization	102,755	103,472
Current liabilities:
Current maturities of long-term debt	486	551
Short-term borrowings	0	0
Payables to affiliates	22,744	22,643
Accounts payable	2,823	2,815
Accrued expenses and other liabilities	3,345	1,789
Total current liabilities	29,398	27,798
Unamortized investment tax credits	0	0
Deferred income taxes	1,413	803
Pension and postretirement benefits other than pensions	0	0
Regulatory liabilities and other	3,242	3,130
Advances for construction	562	541
Contributions in aid of construction	33,877	34,356
TOTAL CAPITALIZATION AND LIABILITIES	$ 171,247	$ 170,100