Equity (Details) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 26, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Apr. 30, 2013 California Water Service Group	Sep. 30, 2013 California Water Service Group	Sep. 30, 2012 California Water Service Group	Sep. 30, 2013 California Water Service Group	Sep. 30, 2012 California Water Service Group	Apr. 30, 2013 Cal Water	Sep. 30, 2013 Cal Water	Sep. 30, 2012 Cal Water	Sep. 30, 2013 Cal Water	Sep. 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				105,600,000
Share-based compensation expense				1,335,000
Common stock dividends declared				(21,981,000)
Net income		29,151,000	29,772,000	41,588,000	43,822,000		29,151,000	29,772,000	41,588,000	43,822,000		28,386,000	29,034,000	42,177,000	43,607,000
Balance at the end of the period		600,254,000		600,254,000			600,254,000		600,254,000			500,359,000		500,359,000
Common stock sold in an underwritten public offering	5,750,000
Proceeds from common stock sold in an underwritten public offering	105,600,000
Underwriting discounts and commissions and offering expenses	5,100,000
Equity
Repayment on line of credit						$ 68,300,000					$ 25,000,000