Condensed Consolidating Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Utility plant:
Utility plant	$ 3,835,194			$ 3,550,485
Less accumulated depreciation and amortization	(1,238,880)			(1,144,115)
Net utility plant	2,596,314			2,406,370
Current assets:
Cash and cash equivalents	113,312			42,653
Receivables and unbilled revenue, net	169,115			119,349
Receivables from affiliates	0			0
Other current assets	23,337			22,710
Total current assets	305,764			184,712
Other assets:
Regulatory assets	484,435			433,322
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	119,921			86,904
Total other assets	604,356			520,226
TOTAL ASSETS	3,506,434			3,111,308
Capitalization:
Common stockholders’ equity	890,192	$ 782,023	$ 755,140	779,906	$ 757,795	$ 723,164	$ 714,129	$ 730,157
Affiliate long-term debt	0			0
Long-term debt, net	785,055			786,754
Total capitalization	1,675,247			1,566,660
Current liabilities:
Current maturities of long-term debt, net	21,883			21,868
Short-term borrowings	375,100			175,100
Payables to affiliates	0			0
Accounts payable	127,158			108,463
Accrued expenses and other liabilities	79,682			53,290
Total current liabilities	603,823			358,721
Deferred income taxes	245,456			222,590
Pension and postretirement benefits other than pensions	261,081			258,907
Regulatory liabilities and other	257,054			271,831
Advances for construction	196,853			191,062
Contributions in aid of construction	266,920			241,537
TOTAL CAPITALIZATION AND LIABILITIES	3,506,434			3,111,308
Consolidating Adjustments
Utility plant:
Utility plant	(7,197)			(7,197)
Less accumulated depreciation and amortization	2,239			2,180
Net utility plant	(4,958)			(5,017)
Current assets:
Cash and cash equivalents	0			0
Receivables and unbilled revenue, net	0			0
Receivables from affiliates	(28,165)			(29,633)
Other current assets	0			0
Total current assets	(28,165)			(29,633)
Other assets:
Regulatory assets	0			0
Investments in affiliates	(917,901)			(777,170)
Long-term affiliate notes receivable	(32,659)			(30,060)
Other assets	(218)			(221)
Total other assets	(950,778)			(807,451)
TOTAL ASSETS	(983,901)			(842,101)
Capitalization:
Common stockholders’ equity	(923,077)			(782,388)
Affiliate long-term debt	(32,659)			(30,060)
Long-term debt, net	0			0
Total capitalization	(955,736)			(812,448)
Current liabilities:
Current maturities of long-term debt, net	0			0
Short-term borrowings	0			0
Payables to affiliates	(28,165)			(29,633)
Accounts payable	0			0
Accrued expenses and other liabilities	0			0
Total current liabilities	(28,165)			(29,633)
Deferred income taxes	0			(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	(983,901)			(842,101)
Parent Company | Reportable Legal Entities
Utility plant:
Utility plant	1,318			1,318
Less accumulated depreciation and amortization	(1,177)			(1,107)
Net utility plant	141			211
Current assets:
Cash and cash equivalents	16,679			3,096
Receivables and unbilled revenue, net	0			0
Receivables from affiliates	26,962			25,803
Other current assets	272			90
Total current assets	43,913			28,989
Other assets:
Regulatory assets	0			0
Investments in affiliates	917,901			777,170
Long-term affiliate notes receivable	32,659			30,060
Other assets	2,349			409
Total other assets	952,909			807,639
TOTAL ASSETS	996,963			836,839
Capitalization:
Common stockholders’ equity	890,192			779,906
Affiliate long-term debt	0			0
Long-term debt, net	0			0
Total capitalization	890,192			779,906
Current liabilities:
Current maturities of long-term debt, net	0			0
Short-term borrowings	105,100			55,100
Payables to affiliates	0			0
Accounts payable	0			0
Accrued expenses and other liabilities	31			313
Total current liabilities	105,131			55,413
Deferred income taxes	1,640			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	996,963			836,839
Cal Water | Reportable Legal Entities
Utility plant:
Utility plant	3,559,329			3,332,331
Less accumulated depreciation and amortization	(1,151,266)			(1,079,627)
Net utility plant	2,408,063			2,252,704
Current assets:
Cash and cash equivalents	85,877			29,098
Receivables and unbilled revenue, net	160,479			114,999
Receivables from affiliates	942			3,621
Other current assets	20,933			20,615
Total current assets	268,231			168,333
Other assets:
Regulatory assets	479,613			428,639
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	84,154			81,591
Total other assets	563,767			510,230
TOTAL ASSETS	3,240,061			2,931,267
Capitalization:
Common stockholders’ equity	805,444			700,784
Affiliate long-term debt	0			0
Long-term debt, net	784,715			786,310
Total capitalization	1,590,159			1,487,094
Current liabilities:
Current maturities of long-term debt, net	21,763			21,732
Short-term borrowings	270,000			120,000
Payables to affiliates	2,940			6,115
Accounts payable	123,129			104,419
Accrued expenses and other liabilities	73,913			50,569
Total current liabilities	491,745			302,835
Deferred income taxes	235,791			217,847
Pension and postretirement benefits other than pensions	261,081			258,907
Regulatory liabilities and other	251,230			264,434
Advances for construction	196,324			190,568
Contributions in aid of construction	213,731			209,582
TOTAL CAPITALIZATION AND LIABILITIES	3,240,061			2,931,267
All Other Subsidiaries | Reportable Legal Entities
Utility plant:
Utility plant	281,744			224,033
Less accumulated depreciation and amortization	(88,676)			(65,561)
Net utility plant	193,068			158,472
Current assets:
Cash and cash equivalents	10,756			10,459
Receivables and unbilled revenue, net	8,636			4,350
Receivables from affiliates	261			209
Other current assets	2,132			2,005
Total current assets	21,785			17,023
Other assets:
Regulatory assets	4,822			4,683
Investments in affiliates	0			0
Long-term affiliate notes receivable	0			0
Other assets	33,636			5,125
Total other assets	38,458			9,808
TOTAL ASSETS	253,311			185,303
Capitalization:
Common stockholders’ equity	117,633			81,604
Affiliate long-term debt	32,659			30,060
Long-term debt, net	340			444
Total capitalization	150,632			112,108
Current liabilities:
Current maturities of long-term debt, net	120			136
Short-term borrowings	0			0
Payables to affiliates	25,225			23,518
Accounts payable	4,029			4,044
Accrued expenses and other liabilities	5,738			2,408
Total current liabilities	35,112			30,106
Deferred income taxes	8,025			3,243
Pension and postretirement benefits other than pensions	0			0
Regulatory liabilities and other	5,824			7,397
Advances for construction	529			494
Contributions in aid of construction	53,189			31,955
TOTAL CAPITALIZATION AND LIABILITIES	$ 253,311			$ 185,303