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Derivative Instruments: (Details) (USD $)
 1 Months Ended 3 Months Ended 6 Months Ended
Jan. 31, 2012
MW
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Derivative instruments          
Regulatory asset not accruing carrying costs   $ 74,300,000   $ 74,300,000  
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 (7,611,000) (1,540,000) (7,506,000) (3,060,000) (7,611,000)
Unrealized gain on purchased power contracts   393,000 2,330,000 1,913,000 2,435,000
Balance, at end of the period   (1,147,000) (5,176,000) (1,147,000) (5,176,000)
Memorandum account to track unrealized gains and losses on purchase power contracts
          
Derivative instruments          
Regulatory asset not accruing carrying costs   $ 1,100,000   $ 1,100,000