Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating activities:
Net income	$ 13,022	$ 18,531	$ 10,477	$ 19,663
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	0	0	0	0
Dividends received from affiliates			0	0
Depreciation and amortization			42,582	39,328
Change in value of life insurance contracts			817	(1,208)
Allowance for equity funds used during construction	(710)	(879)	(1,621)	(1,658)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			(16,389)	(40,086)
Other changes in noncurrent assets and liabilities			13,705	19,687
Net cash provided by operating activities			49,571	35,726
Investing activities:
Utility plant expenditures			(133,900)	(108,712)
Changes in affiliate advances			0	0
Issuance of affiliate short-term borrowings			0	0
Reduction of affiliates long-term debt			0	0
Life insurance proceeds			2,054	450
Purchase of life insurance contracts			(2,054)	(1,216)
Net cash used in investing activities			(133,900)	(109,478)
Financing Activities:
Short-term borrowings			111,000	140,000
Repayment of short-term borrowings			(61,000)	(47,000)
Proceeds from affiliate short-term borrowings			0	0
Changes in affiliate advances			0	0
Repayment of affiliates long-term borrowings			0	0
Repayment of long-term debt			(12,264)	(2,407)
Advances and contributions in aid of construction			8,385	10,312
Refunds of advances for construction			(3,498)	(4,430)
Repurchase of common stock			(1,364)	(1,236)
Dividends paid to non-affiliates			(18,017)	(17,278)
Dividends paid to affiliates			0	0
Net cash provided by financing activities			23,242	77,961
Change in cash, cash equivalents, and restricted cash			(61,087)	4,209
Cash, cash equivalents, and restricted cash at beginning of period			95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	34,265	30,144	34,265	30,144
Consolidating Adjustments
Operating activities:
Net income	(12,917)	(18,402)	(10,177)	(19,387)
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	12,934	18,415	10,209	19,413
Dividends received from affiliates			(18,017)	(17,278)
Depreciation and amortization			(43)	(46)
Change in value of life insurance contracts			0	0
Allowance for equity funds used during construction	0	0	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			0	0
Other changes in noncurrent assets and liabilities			11	20
Net cash provided by operating activities			(18,017)	(17,278)
Investing activities:
Utility plant expenditures			0	0
Changes in affiliate advances			(3,187)	(2,782)
Issuance of affiliate short-term borrowings			3,700	2,610
Reduction of affiliates long-term debt			(818)	(668)
Life insurance proceeds			0	0
Purchase of life insurance contracts			0	0
Net cash used in investing activities			(305)	(840)
Financing Activities:
Short-term borrowings			0	0
Repayment of short-term borrowings			0	0
Proceeds from affiliate short-term borrowings			(3,700)	(2,610)
Changes in affiliate advances			3,187	2,782
Repayment of affiliates long-term borrowings			818	668
Repayment of long-term debt			0	0
Advances and contributions in aid of construction			0	0
Refunds of advances for construction			0	0
Repurchase of common stock			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			18,017	17,278
Net cash provided by financing activities			18,322	18,118
Change in cash, cash equivalents, and restricted cash			0	0
Cash, cash equivalents, and restricted cash at beginning of period			0
Cash, cash equivalents, and restricted cash at end of period	0		0
Parent Company | Reportable Legal Entities
Operating activities:
Net income	13,022	18,531	10,477	19,663
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	(12,934)	(18,415)	(10,209)	(19,413)
Dividends received from affiliates			18,017	17,278
Depreciation and amortization			47	47
Change in value of life insurance contracts			0	0
Allowance for equity funds used during construction	0	0	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			(417)	(172)
Other changes in noncurrent assets and liabilities			1,610	1,520
Net cash provided by operating activities			19,525	18,923
Investing activities:
Utility plant expenditures			0	0
Changes in affiliate advances			58	429
Issuance of affiliate short-term borrowings			(3,700)	(2,610)
Reduction of affiliates long-term debt			818	668
Life insurance proceeds			0	0
Purchase of life insurance contracts			0	0
Net cash used in investing activities			(2,824)	(1,513)
Financing Activities:
Short-term borrowings			0	0
Repayment of short-term borrowings			0	(2,000)
Proceeds from affiliate short-term borrowings			0	0
Changes in affiliate advances			0	0
Repayment of affiliates long-term borrowings			0	0
Repayment of long-term debt			0	0
Advances and contributions in aid of construction			0	0
Refunds of advances for construction			0	0
Repurchase of common stock			(1,364)	(1,236)
Dividends paid to non-affiliates			(18,017)	(17,278)
Dividends paid to affiliates			0	0
Net cash provided by financing activities			(19,381)	(20,514)
Change in cash, cash equivalents, and restricted cash			(2,680)	(3,104)
Cash, cash equivalents, and restricted cash at beginning of period			4,728
Cash, cash equivalents, and restricted cash at end of period	2,048		2,048
Cal Water | Reportable Legal Entities
Operating activities:
Net income	11,896	17,660	9,176	18,744
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	0	0	0	0
Dividends received from affiliates			0	0
Depreciation and amortization			40,146	37,074
Change in value of life insurance contracts			817	(1,208)
Allowance for equity funds used during construction	(710)	(879)	(1,621)	(1,658)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			(16,439)	(40,755)
Other changes in noncurrent assets and liabilities			11,653	17,650
Net cash provided by operating activities			43,732	29,847
Investing activities:
Utility plant expenditures			(128,683)	(105,684)
Changes in affiliate advances			3,323	2,491
Issuance of affiliate short-term borrowings			0	0
Reduction of affiliates long-term debt			0	0
Life insurance proceeds			2,054	450
Purchase of life insurance contracts			(2,054)	(1,216)
Net cash used in investing activities			(125,360)	(103,959)
Financing Activities:
Short-term borrowings			111,000	140,000
Repayment of short-term borrowings			(61,000)	(45,000)
Proceeds from affiliate short-term borrowings			0	0
Changes in affiliate advances			129	(238)
Repayment of affiliates long-term borrowings			0	0
Repayment of long-term debt			(12,136)	(2,161)
Advances and contributions in aid of construction			8,029	10,225
Refunds of advances for construction			(3,489)	(4,430)
Repurchase of common stock			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			(17,330)	(16,484)
Net cash provided by financing activities			25,203	81,912
Change in cash, cash equivalents, and restricted cash			(56,425)	7,800
Cash, cash equivalents, and restricted cash at beginning of period			81,453
Cash, cash equivalents, and restricted cash at end of period	25,028		25,028
All Other Subsidiaries | Reportable Legal Entities
Operating activities:
Net income	1,021	742	1,001	643
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	0	0	0	0
Dividends received from affiliates			0	0
Depreciation and amortization			2,432	2,253
Change in value of life insurance contracts			0	0
Allowance for equity funds used during construction	0	$ 0	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			467	841
Other changes in noncurrent assets and liabilities			431	497
Net cash provided by operating activities			4,331	4,234
Investing activities:
Utility plant expenditures			(5,217)	(3,028)
Changes in affiliate advances			(194)	(138)
Issuance of affiliate short-term borrowings			0	0
Reduction of affiliates long-term debt			0	0
Life insurance proceeds			0	0
Purchase of life insurance contracts			0	0
Net cash used in investing activities			(5,411)	(3,166)
Financing Activities:
Short-term borrowings			0	0
Repayment of short-term borrowings			0	0
Proceeds from affiliate short-term borrowings			3,700	2,610
Changes in affiliate advances			(3,316)	(2,544)
Repayment of affiliates long-term borrowings			(818)	(668)
Repayment of long-term debt			(128)	(246)
Advances and contributions in aid of construction			356	87
Refunds of advances for construction			(9)	0
Repurchase of common stock			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			(687)	(794)
Net cash provided by financing activities			(902)	(1,555)
Change in cash, cash equivalents, and restricted cash			(1,982)	$ (487)
Cash, cash equivalents, and restricted cash at beginning of period			9,171
Cash, cash equivalents, and restricted cash at end of period	$ 7,189		$ 7,189