Equity (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 26, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Apr. 30, 2013 California Water Service Group	Jun. 30, 2013 California Water Service Group	Jun. 30, 2012 California Water Service Group	Jun. 30, 2013 California Water Service Group	Jun. 30, 2012 California Water Service Group	Apr. 30, 2013 Cal Water	Jun. 30, 2013 Cal Water	Jun. 30, 2012 Cal Water	Jun. 30, 2013 Cal Water	Jun. 30, 2012 Cal Water
Changes in total stockholders' equity
Balance at the beginning of the period				$ 473,712,000					$ 473,712,000					$ 442,923,000
Common stock issued, net				105,835,000
Share-based compensation expense				839,000
Common stock dividends declared				(14,343,000)
Net income		13,510,000	12,965,000	12,437,000	14,050,000		13,510,000	12,965,000	12,437,000	14,050,000		14,048,000	13,025,000	13,791,000	14,573,000
Balance at the end of the period		578,480,000		578,480,000			578,480,000		578,480,000			478,802,000		478,802,000
Common stock sold in an underwritten public offering	5,750,000
Proceeds from common stock sold in an underwritten public offering	105,800,000
Underwriting discounts and commissions and offering expenses	4,900,000
Equity
Repayment on line of credit						$ 68,300,000					$ 25,000,000