Condensed Consolidating Financial Statements - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities:
Net (loss) income	$ (2,545)	$ 1,132
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity loss of subsidiaries	0	0
Dividends received from affiliates	0	0
Depreciation and amortization	21,207	19,658
Change in value of life insurance contracts	1,137	(319)
Allowance for equity funds used during construction	(911)	(779)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities	3,081	(10,374)
Other changes in noncurrent assets and liabilities	6,283	4,979
Net cash provided by operating activities	28,252	14,297
Investing activities:
Utility plant expenditures	(70,650)	(51,853)
Changes in affiliate advances	0	0
Issuance of affiliate short-term borrowings		0
Reduction of affiliates long-term debt	0	0
Life insurance proceeds	0	450
Purchase of life insurance contracts	0	(836)
Net cash used in investing activities	(70,650)	(52,239)
Financing Activities:
Short-term borrowings	45,022	35,000
Repayment of short-term borrowings	(45,022)	(2,000)
Proceeds from affiliate short-term borrowings		0
Changes in affiliate advances	0	0
Repayment of affiliates long-term borrowings	0	0
Repayment of long-term debt	(10,224)	(286)
Advances and contributions in aid of construction	4,763	3,975
Refunds of advances for construction	(1,918)	(2,236)
Repurchase of common stock	(1,239)	(1,119)
Dividends paid to non-affiliates	(9,003)	(8,634)
Dividends paid to affiliates	0	0
Net cash (used in) provided by financing activities	(17,621)	24,700
Change in cash, cash equivalents, and restricted cash	(60,019)	(13,242)
Cash, cash equivalents, and restricted cash at beginning of period	95,352	25,935
Cash, cash equivalents, and restricted cash at end of period	35,333	12,693
Consolidating Adjustments
Operating activities:
Net (loss) income	2,740	(985)
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity loss of subsidiaries	(2,725)	998
Dividends received from affiliates	(9,003)	(8,634)
Depreciation and amortization	(22)	(23)
Change in value of life insurance contracts	0	0
Allowance for equity funds used during construction	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities	0	0
Other changes in noncurrent assets and liabilities	7	10
Net cash provided by operating activities	(9,003)	(8,634)
Investing activities:
Utility plant expenditures	0	0
Changes in affiliate advances	(562)	(1,373)
Issuance of affiliate short-term borrowings		325
Reduction of affiliates long-term debt	(395)	(332)
Life insurance proceeds		0
Purchase of life insurance contracts		0
Net cash used in investing activities	(957)	(1,380)
Financing Activities:
Short-term borrowings	0	0
Repayment of short-term borrowings	0	0
Proceeds from affiliate short-term borrowings		(325)
Changes in affiliate advances	562	1,373
Repayment of affiliates long-term borrowings	395	332
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	9,003	8,634
Net cash (used in) provided by financing activities	9,960	10,014
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
Parent Company | Reportable Legal Entities
Operating activities:
Net (loss) income	(2,545)	1,132
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity loss of subsidiaries	2,725	(998)
Dividends received from affiliates	9,003	8,634
Depreciation and amortization	23	23
Change in value of life insurance contracts	0	0
Allowance for equity funds used during construction	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities	(376)	(67)
Other changes in noncurrent assets and liabilities	806	483
Net cash provided by operating activities	9,636	9,207
Investing activities:
Utility plant expenditures	0	0
Changes in affiliate advances	(2,520)	593
Issuance of affiliate short-term borrowings		(325)
Reduction of affiliates long-term debt	395	332
Life insurance proceeds		0
Purchase of life insurance contracts		0
Net cash used in investing activities	(2,125)	600
Financing Activities:
Short-term borrowings	0	0
Repayment of short-term borrowings	0	(2,000)
Proceeds from affiliate short-term borrowings		0
Changes in affiliate advances	245	715
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	(1,239)	(1,119)
Dividends paid to non-affiliates	(9,003)	(8,634)
Dividends paid to affiliates	0	0
Net cash (used in) provided by financing activities	(9,997)	(11,038)
Change in cash, cash equivalents, and restricted cash	(2,486)	(1,231)
Cash, cash equivalents, and restricted cash at beginning of period	4,728
Cash, cash equivalents, and restricted cash at end of period	2,242
Cal Water | Reportable Legal Entities
Operating activities:
Net (loss) income	(2,720)	1,084
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity loss of subsidiaries	0	0
Dividends received from affiliates	0	0
Depreciation and amortization	20,081	18,542
Change in value of life insurance contracts	1,137	(319)
Allowance for equity funds used during construction	(911)	(779)
Changes in operating assets and liabilities:
Changes in operating assets and liabilities	1,728	(11,003)
Other changes in noncurrent assets and liabilities	5,368	4,241
Net cash provided by operating activities	24,683	11,766
Investing activities:
Utility plant expenditures	(67,841)	(50,509)
Changes in affiliate advances	3,235	955
Issuance of affiliate short-term borrowings		0
Reduction of affiliates long-term debt	0	0
Life insurance proceeds		450
Purchase of life insurance contracts		(836)
Net cash used in investing activities	(64,606)	(49,940)
Financing Activities:
Short-term borrowings	45,022	35,000
Repayment of short-term borrowings	(45,022)	0
Proceeds from affiliate short-term borrowings		0
Changes in affiliate advances	(423)	(475)
Repayment of affiliates long-term borrowings	0	0
Repayment of long-term debt	(10,158)	(170)
Advances and contributions in aid of construction	4,663	3,952
Refunds of advances for construction	(1,908)	(2,236)
Repurchase of common stock	0	0
Dividends paid to non-affiliates	0	0
Dividends paid to affiliates	(8,665)	(8,234)
Net cash (used in) provided by financing activities	(16,491)	27,837
Change in cash, cash equivalents, and restricted cash	(56,414)	(10,337)
Cash, cash equivalents, and restricted cash at beginning of period	81,453
Cash, cash equivalents, and restricted cash at end of period	25,039
All Other Subsidiaries | Reportable Legal Entities
Operating activities:
Net (loss) income	(20)	(99)
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity loss of subsidiaries	0	0
Dividends received from affiliates	0	0
Depreciation and amortization	1,125	1,116
Change in value of life insurance contracts	0	0
Allowance for equity funds used during construction	0	0
Changes in operating assets and liabilities:
Changes in operating assets and liabilities	1,729	696
Other changes in noncurrent assets and liabilities	102	245
Net cash provided by operating activities	2,936	1,958
Investing activities:
Utility plant expenditures	(2,809)	(1,344)
Changes in affiliate advances	(153)	(175)
Issuance of affiliate short-term borrowings		0
Reduction of affiliates long-term debt	0	0
Life insurance proceeds		0
Purchase of life insurance contracts		0
Net cash used in investing activities	(2,962)	(1,519)
Financing Activities:
Short-term borrowings	0	0
Repayment of short-term borrowings	0	0
Proceeds from affiliate short-term borrowings		325
Changes in affiliate advances	(384)	(1,613)
Repayment of affiliates long-term borrowings	(395)	(332)
Repayment of long-term debt	(66)	(116)
Advances and contributions in aid of construction	100	23
Refunds of advances for construction	(10)	0
Repurchase of common stock	0	0
Dividends paid to non-affiliates	0	0
Dividends paid to affiliates	(338)	(400)
Net cash (used in) provided by financing activities	(1,093)	(2,113)
Change in cash, cash equivalents, and restricted cash	(1,119)	$ (1,674)
Cash, cash equivalents, and restricted cash at beginning of period	9,171
Cash, cash equivalents, and restricted cash at end of period	$ 8,052