Short-term and Long-term Borrowings (Details) - Revolving Credit Facility - USD ($) $ in Millions	Mar. 10, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Short-term and Long-term Borrowings
Maximum borrowing capacity	$ 450.0
Debt maturity period	5 years
Minimum
Short-term and Long-term Borrowings
Commitment fee (as a percent)	0.08%
Minimum | LIBOR
Short-term and Long-term Borrowings
Interest rate margin (as a percent)	0.725%
Maximum
Short-term and Long-term Borrowings
Commitment fee (as a percent)	0.125%
Maximum | LIBOR
Short-term and Long-term Borrowings
Interest rate margin (as a percent)	0.95%
Parent Company
Short-term and Long-term Borrowings
Maximum borrowing capacity	$ 150.0
Amount outstanding under line of credit		$ 64.2	$ 61.7
Cal Water
Short-term and Long-term Borrowings
Maximum borrowing capacity	$ 300.0
Period within which borrowings are to be repaid	12 months
Additional capacity available	$ 50.0
Amount outstanding under line of credit		$ 72.4	$ 17.4
Average borrowing rate (as a percent)		1.05%		1.16%