Derivative Instruments (Details) (USD $)	1 Months Ended	12 Months Ended
	Jan. 31, 2012 item	Dec. 31, 2012	Dec. 31, 2011
Derivative instruments
Regulatory asset not accruing carrying costs		$ 78,500,000
Purchase power contract
Derivative instruments
Number of broker quotes received in order to determine the fair value of derivative instrument		1
GSWC | Purchase power contract
Derivative instruments
Number of megawatts of base load energy that the entity will be able to purchase under the new agreement	12
Changes in the fair value of the derivative
Balance, at beginning of the period		(7,611,000)	(6,850,000)
Unrealized gain (loss) on purchased power contracts		4,551,000	(761,000)
Balance, at end of the period		(3,060,000)	(7,611,000)
Memorandum account to track unrealized gains and losses on purchase power contracts
Derivative instruments
Regulatory asset not accruing carrying costs		$ 3,100,000