Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)		50,334,000			50,334,000
Balance at beginning of period	$ 929,802	$ 921,344	$ 755,140	$ 779,906	$ 921,344	$ 779,906
Loss attributable to noncontrolling interests	(149)		0		(149)	0
Net income attributable to California Water Service Group	38,225	(3,032)	5,281	(20,307)	$ 35,193	(15,026)
Issuance of common stock	40,902	24,486	32,063	7,229
Repurchase of common stock	(109)	(1,415)	(105)	(1,373)
Dividends paid on common stock	(11,702)	(11,581)	(10,356)	(10,315)
Acquisition of business with noncontrolling interest	$ 5,294
Balance at end of period (in shares)	51,535,000				51,535,000
Balance at end of period	$ 1,002,263	$ 929,802	$ 782,023	$ 755,140	$ 1,002,263	$ 782,023
Dividends paid on common stock (in dollars per share)	$ 0.2300	$ 0.2300	$ 0.2125	$ 0.2125
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period (in shares)	50,835,000	50,334,000	48,714,000	48,532,000	50,334,000	48,532,000
Balance at beginning of period	$ 508	$ 503	$ 487	$ 485	$ 503	$ 485
Issuance of common stock (in shares)	702,000	528,000	686,000	210,000
Issuance of common stock	$ 7	$ 5	$ 7	$ 2
Repurchase of common stock (in shares)	(2,000)	(27,000)	(2,000)	(28,000)
Balance at end of period (in shares)	51,535,000	50,835,000	49,398,000	48,714,000	51,535,000	49,398,000
Balance at end of period	$ 515	$ 508	$ 494	$ 487	$ 515	$ 494
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	471,698	448,632	368,129	362,275	448,632	362,275
Issuance of common stock	40,895	24,481	32,056	7,227
Repurchase of common stock	(109)	(1,415)	(105)	(1,373)
Balance at end of period	512,484	471,698	400,080	368,129	512,484	400,080
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	457,596	472,209	386,524	417,146	472,209	417,146
Net income attributable to California Water Service Group	38,225	(3,032)	5,281	(20,307)
Dividends paid on common stock	(11,702)	(11,581)	(10,356)	(10,315)
Balance at end of period	484,119	457,596	$ 381,449	$ 386,524	484,119	$ 381,449
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	0	0			0
Loss attributable to noncontrolling interests	(149)
Acquisition of business with noncontrolling interest	5,294
Balance at end of period	$ 5,145	$ 0			$ 5,145