Short-term and Long-term Borrowings (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		6 Months Ended						0 Months Ended
	Jun. 30, 2013	Jun. 29, 2011	Jun. 30, 2013 Minimum	Jun. 30, 2013 Maximum	Jun. 30, 2013 California Water Service Group	Dec. 31, 2012 California Water Service Group	Jun. 30, 2012 California Water Service Group	Jun. 29, 2011 California Water Service Group	Jun. 29, 2011 Cal Water	Jun. 30, 2013 Cal Water	Dec. 31, 2012 Cal Water
Short-term borrowings
Maximum borrowing capacity		$ 400						$ 100	$ 300
Period within which borrowings are to be repaid									12 months
Basis of interest rate	LIBOR
Interest rate margin (as a percent)			0.725%	0.95%
Commitment fee (as a percent)			0.08%	0.125%
Amount outstanding under line of credit					$ 8.8	$ 64.5				$ 20.0	$ 25.0
Average borrowing rate (as a percent)					2.30%		1.60%