Condensed Consolidating Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Utility plant:
Utility plant	$ 3,616,418	$ 3,550,485
Less accumulated depreciation and amortization	(1,169,447)	(1,144,115)
Net utility plant	2,446,971	2,406,370
Current assets:
Cash and cash equivalents	140,406	42,653
Receivables and unbilled revenue, net	103,276	119,349
Receivables from affiliates	0	0
Other current assets	26,400	22,710
Total current assets	270,082	184,712
Other assets:
Regulatory assets	442,819	433,322
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	80,973	86,904
Total other assets	523,792	520,226
TOTAL ASSETS	3,240,845	3,111,308
Capitalization:
Common stockholders’ equity	755,140	779,906	$ 714,129	$ 730,157
Affiliate long-term debt	0	0
Long-term debt, net	786,467	786,754
Total capitalization	1,541,607	1,566,660
Current liabilities:
Current maturities of long-term debt, net	21,864	21,868
Short-term borrowings	335,100	175,100
Payables to affiliates	0	0
Accounts payable	99,019	108,463
Accrued expenses and other liabilities	57,144	53,290
Total current liabilities	513,127	358,721
Unamortized investment tax credits	1,575	1,575
Deferred income taxes	218,451	222,590
Pension and postretirement benefits other than pensions	260,337	258,907
Regulatory liabilities and other	268,917	270,256
Advances for construction	194,046	191,062
Contributions in aid of construction	242,785	241,537
TOTAL CAPITALIZATION AND LIABILITIES	3,240,845	3,111,308
Consolidating Adjustments
Utility plant:
Utility plant	(7,197)	(7,197)
Less accumulated depreciation and amortization	2,199	2,180
Net utility plant	(4,998)	(5,017)
Current assets:
Cash and cash equivalents	0	0
Receivables and unbilled revenue, net	0	0
Receivables from affiliates	(24,940)	(29,633)
Other current assets	0	0
Total current assets	(24,940)	(29,633)
Other assets:
Regulatory assets	0	0
Investments in affiliates	(752,501)	(777,170)
Long-term affiliate notes receivable	(29,579)	(30,060)
Other assets	(221)	(221)
Total other assets	(782,301)	(807,451)
TOTAL ASSETS	(812,239)	(842,101)
Capitalization:
Common stockholders’ equity	(757,705)	(782,388)
Affiliate long-term debt	(29,579)	(30,060)
Long-term debt, net	0	0
Total capitalization	(787,284)	(812,448)
Current liabilities:
Current maturities of long-term debt, net	0	0
Short-term borrowings	0	0
Payables to affiliates	(24,940)	(29,633)
Accounts payable	0	0
Accrued expenses and other liabilities	0	0
Total current liabilities	(24,940)	(29,633)
Unamortized investment tax credits	0	0
Deferred income taxes	(15)	(20)
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	(812,239)	(842,101)
Parent Company | Reportable Legal Entities
Utility plant:
Utility plant	1,318	1,318
Less accumulated depreciation and amortization	(1,130)	(1,107)
Net utility plant	188	211
Current assets:
Cash and cash equivalents	56,356	3,096
Receivables and unbilled revenue, net	0	0
Receivables from affiliates	23,212	25,803
Other current assets	634	90
Total current assets	80,202	28,989
Other assets:
Regulatory assets	0	0
Investments in affiliates	752,501	777,170
Long-term affiliate notes receivable	29,579	30,060
Other assets	458	409
Total other assets	782,538	807,639
TOTAL ASSETS	862,928	836,839
Capitalization:
Common stockholders’ equity	755,140	779,906
Affiliate long-term debt	0	0
Long-term debt, net	0	0
Total capitalization	755,140	779,906
Current liabilities:
Current maturities of long-term debt, net	0	0
Short-term borrowings	105,100	55,100
Payables to affiliates	797	0
Accounts payable	0	0
Accrued expenses and other liabilities	334	313
Total current liabilities	106,231	55,413
Unamortized investment tax credits	0	0
Deferred income taxes	1,557	1,520
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	862,928	836,839
Cal Water | Reportable Legal Entities
Utility plant:
Utility plant	3,395,699	3,332,331
Less accumulated depreciation and amortization	(1,103,286)	(1,079,627)
Net utility plant	2,292,413	2,252,704
Current assets:
Cash and cash equivalents	74,655	29,098
Receivables and unbilled revenue, net	98,794	114,999
Receivables from affiliates	1,704	3,621
Other current assets	23,251	20,615
Total current assets	198,404	168,333
Other assets:
Regulatory assets	438,075	428,639
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	75,228	81,591
Total other assets	513,303	510,230
TOTAL ASSETS	3,004,120	2,931,267
Capitalization:
Common stockholders’ equity	676,674	700,784
Affiliate long-term debt	0	0
Long-term debt, net	786,040	786,310
Total capitalization	1,462,714	1,487,094
Current liabilities:
Current maturities of long-term debt, net	21,743	21,732
Short-term borrowings	230,000	120,000
Payables to affiliates	25	6,115
Accounts payable	94,979	104,419
Accrued expenses and other liabilities	53,721	50,569
Total current liabilities	400,468	302,835
Unamortized investment tax credits	1,575	1,575
Deferred income taxes	213,617	217,847
Pension and postretirement benefits other than pensions	260,337	258,907
Regulatory liabilities and other	261,072	262,859
Advances for construction	193,568	190,568
Contributions in aid of construction	210,769	209,582
TOTAL CAPITALIZATION AND LIABILITIES	3,004,120	2,931,267
All Other Subsidiaries | Reportable Legal Entities
Utility plant:
Utility plant	226,598	224,033
Less accumulated depreciation and amortization	(67,230)	(65,561)
Net utility plant	159,368	158,472
Current assets:
Cash and cash equivalents	9,395	10,459
Receivables and unbilled revenue, net	4,482	4,350
Receivables from affiliates	24	209
Other current assets	2,515	2,005
Total current assets	16,416	17,023
Other assets:
Regulatory assets	4,744	4,683
Investments in affiliates	0	0
Long-term affiliate notes receivable	0	0
Other assets	5,508	5,125
Total other assets	10,252	9,808
TOTAL ASSETS	186,036	185,303
Capitalization:
Common stockholders’ equity	81,031	81,604
Affiliate long-term debt	29,579	30,060
Long-term debt, net	427	444
Total capitalization	111,037	112,108
Current liabilities:
Current maturities of long-term debt, net	121	136
Short-term borrowings	0	0
Payables to affiliates	24,118	23,518
Accounts payable	4,040	4,044
Accrued expenses and other liabilities	3,089	2,408
Total current liabilities	31,368	30,106
Unamortized investment tax credits	0	0
Deferred income taxes	3,292	3,243
Pension and postretirement benefits other than pensions	0	0
Regulatory liabilities and other	7,845	7,397
Advances for construction	478	494
Contributions in aid of construction	32,016	31,955
TOTAL CAPITALIZATION AND LIABILITIES	$ 186,036	$ 185,303