Derivative Instruments: (Details) (USD $)		3 Months Ended		1 Months Ended	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Purchase power contract quote	Mar. 31, 2012 Purchase power contract	Jan. 31, 2012 GSWC Purchase power contract item	Mar. 31, 2013 GSWC Purchase power contract	Mar. 31, 2013 Memorandum account to track unrealized gains and losses on purchase power contracts
Derivative instruments
Impact on earnings					$ 0
Regulatory asset not accruing carrying costs	75,400,000					1,500,000
Number of megawatts of base load energy that the entity will be able to purchase under the new agreement				12
Number of broker quotes received in order to determine the fair value of derivative instrument		1
Changes in the fair value of the derivative
Balance, at beginning of the period		(3,060,000)	(7,611,000)
Unrealized gain on purchased power contracts		1,520,000	105,000
Balance, at end of the period		$ (1,540,000)	$ (7,506,000)