Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating activities:
Net income	$ 34,392	$ 33,849	$ 44,869	$ 53,512
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			64,131	59,016
Change in value of life insurance contracts			124	(1,871)
Allowance for equity funds used during construction	(1,023)	(1,105)	(2,644)	(2,763)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			7,032	(39,943)
Other changes in noncurrent assets and liabilities			5,991	41,837
Net cash provided by operating activities			119,503	109,788
Investing activities:
Utility plant expenditures			(212,856)	(180,442)
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			3,491	1,558
Purchase of life insurance contracts			(4,925)	(3,948)
Net cash used in investing activities			(214,290)	(182,832)
Financing Activities:
Short-term borrowings			141,000	185,000
Repayment of short-term borrowings			(341,000)	(87,000)
Proceeds from affiliate short-term borrowings			0	0
Changes in affiliate advances			0	0
Repayment of affiliates long-term borrowings			0	0
Proceeds from Issuance of Long-term Debt			299,383	0
Repayment of long-term debt			(12,499)	(2,797)
Advances and contributions in aid of construction			13,630	14,964
Refunds of advances for construction			(5,462)	(6,316)
Repurchase of common stock			(1,496)	(1,359)
Dividends paid to non-affiliates			(27,029)	(25,920)
Dividends paid to affiliates			0	0
Net cash provided by financing activities			66,527	76,572
Change in cash, cash equivalents, and restricted cash			(28,260)	3,528
Cash, cash equivalents, and restricted cash at beginning of period			95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	67,092	29,463	67,092	29,463
Consolidating Adjustments
Operating activities:
Net income	(34,292)	(33,734)	(44,469)	(53,121)
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	34,307	33,748	44,516	53,161
Dividends received from affiliates			(27,029)	(25,920)
Depreciation and amortization			(65)	(69)
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			18	29
Net cash provided by operating activities			(27,029)	(25,920)
Investing activities:
Utility plant expenditures			0	0
Changes in affiliate advances			(1,954)	(2,284)
Issuance of affiliate short-term borrowings			23,700	2,610
Reduction of affiliates long-term debt			(1,224)	(1,010)
Life insurance proceeds			0	0
Purchase of life insurance contracts			0	0
Net cash used in investing activities			20,522	(684)
Financing Activities:
Short-term borrowings			0	0
Repayment of short-term borrowings			0	0
Proceeds from affiliate short-term borrowings			(23,700)	(2,610)
Changes in affiliate advances			1,954	2,284
Repayment of affiliates long-term borrowings			1,224	1,010
Proceeds from Issuance of Long-term Debt			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			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			27,029	25,920
Net cash provided by financing activities			6,507	26,604
Change in cash, cash equivalents, and restricted cash			0	0
Cash, cash equivalents, and restricted cash at beginning of period			0	0
Cash, cash equivalents, and restricted cash at end of period	0	0	0	0
Parent Company | Reportable Legal Entities
Operating activities:
Net income	34,392	33,849	44,869	53,512
Adjustments to reconcile net income to net cash provided by operating activities:
Equity earnings of subsidiaries	(34,307)	(33,748)	(44,516)	(53,161)
Dividends received from affiliates			27,029	25,920
Depreciation and amortization			70	70
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			(281)	(38)
Other changes in noncurrent assets and liabilities			2,518	2,420
Net cash provided by operating activities			29,689	28,723
Investing activities:
Utility plant expenditures			4	(4)
Changes in affiliate advances			(975)	(334)
Issuance of affiliate short-term borrowings			(23,700)	(2,610)
Reduction of affiliates long-term debt			1,224	1,010
Life insurance proceeds			0	0
Purchase of life insurance contracts			0	0
Net cash used in investing activities			(23,447)	(1,938)
Financing Activities:
Short-term borrowings			20,000	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
Proceeds from Issuance of Long-term Debt			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			(1,496)	(1,359)
Dividends paid to non-affiliates			(27,029)	(25,920)
Dividends paid to affiliates			0	0
Net cash provided by financing activities			(8,525)	(29,279)
Change in cash, cash equivalents, and restricted cash			(2,283)	(2,494)
Cash, cash equivalents, and restricted cash at beginning of period			4,728	5,216
Cash, cash equivalents, and restricted cash at end of period	2,445	2,722	2,445	2,722
Cal Water | Reportable Legal Entities
Operating activities:
Net income	33,072	31,906	42,248	50,650
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			60,298	55,623
Change in value of life insurance contracts			124	(1,871)
Allowance for equity funds used during construction	(1,023)	(1,105)	(2,644)	(2,763)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities			6,135	(40,941)
Other changes in noncurrent assets and liabilities			1,466	37,125
Net cash provided by operating activities			107,627	97,823
Investing activities:
Utility plant expenditures			(205,218)	(175,234)
Changes in affiliate advances			3,198	2,905
Issuance of affiliate short-term borrowings			0	0
Reduction of affiliates long-term debt			0	0
Life insurance proceeds			3,491	1,558
Purchase of life insurance contracts			(4,925)	(3,948)
Net cash used in investing activities			(203,454)	(174,719)
Financing Activities:
Short-term borrowings			121,000	185,000
Repayment of short-term borrowings			(341,000)	(85,000)
Proceeds from affiliate short-term borrowings			20,000	0
Changes in affiliate advances			1,129	718
Repayment of affiliates long-term borrowings			0	0
Proceeds from Issuance of Long-term Debt			299,383
Repayment of long-term debt			(12,299)	(2,336)
Advances and contributions in aid of construction			13,288	14,900
Refunds of advances for construction			(5,452)	(6,311)
Repurchase of common stock			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			(25,959)	(24,760)
Net cash provided by financing activities			70,090	82,211
Change in cash, cash equivalents, and restricted cash			(25,737)	5,315
Cash, cash equivalents, and restricted cash at beginning of period			81,453	13,595
Cash, cash equivalents, and restricted cash at end of period	55,716	18,910	55,716	18,910
All Other Subsidiaries | Reportable Legal Entities
Operating activities:
Net income	1,220	1,828	2,221	2,471
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			3,828	3,392
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			1,178	1,036
Other changes in noncurrent assets and liabilities			1,989	2,263
Net cash provided by operating activities			9,216	9,162
Investing activities:
Utility plant expenditures			(7,642)	(5,204)
Changes in affiliate advances			(269)	(287)
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			(7,911)	(5,491)
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			(3,083)	(3,002)
Repayment of affiliates long-term borrowings			(1,224)	(1,010)
Proceeds from Issuance of Long-term Debt			0
Repayment of long-term debt			(200)	(461)
Advances and contributions in aid of construction			342	64
Refunds of advances for construction			(10)	(5)
Repurchase of common stock			0	0
Dividends paid to non-affiliates			0	0
Dividends paid to affiliates			(1,070)	(1,160)
Net cash provided by financing activities			(1,545)	(2,964)
Change in cash, cash equivalents, and restricted cash			(240)	707
Cash, cash equivalents, and restricted cash at beginning of period			9,171	7,124
Cash, cash equivalents, and restricted cash at end of period	$ 8,931	$ 7,831	$ 8,931	$ 7,831