Short-term Borrowings (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		9 Months Ended						9 Months Ended
	Sep. 30, 2012	Jun. 29, 2011	Sep. 30, 2012 Minimum	Sep. 30, 2012 Maximum	Sep. 30, 2012 California Water Service Group	Dec. 31, 2011 California Water Service Group	Sep. 30, 2011 California Water Service Group	Jun. 29, 2011 California Water Service Group	Sep. 30, 2012 Cal Water month	Jun. 29, 2011 Cal Water
Short-term borrowings
Maximum borrowing capacity		$ 400						$ 100		$ 300
Basis of interest rate	LIBOR
Interest rate margin (as a percent)			0.725%	0.95%
Commitment fee (as a percent)			0.08%	0.125%
Period within which borrowings are to be repaid (in months)									12
Amount outstanding under line of credit					$ 60.7	$ 47.1
Average borrowing rate (as a percent)					1.70%		2.60%