Derivative Instruments: (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 MW	Sep. 30, 2012
Derivative instruments
Regulatory asset not accruing carrying costs	$ 72,300,000		$ 72,300,000
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
Impact on earnings			0
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	(1,147,000)	(5,176,000)	(3,060,000)	(7,611,000)
Unrealized gain on purchased power contracts	559,000	2,457,000	2,472,000	4,892,000
Balance, at end of the period	(588,000)	(2,719,000)	(588,000)	(2,719,000)
Memorandum account to track unrealized gains and losses on purchase power contracts
Derivative instruments
Regulatory asset not accruing carrying costs	$ 588,000		$ 588,000