Derivative instruments: (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended				12 Months Ended						12 Months Ended
	Dec. 31, 2011 Designated as hedges	Dec. 31, 2010 Designated as hedges	Dec. 31, 2011 Designated as hedges Natural gas swaps Accounts receivable	Dec. 31, 2010 Designated as hedges Natural gas swaps Accounts receivable	Dec. 31, 2011 Designated as hedges Natural gas swaps Fuel	Dec. 31, 2010 Designated as hedges Natural gas swaps Fuel	Dec. 31, 2012 Not designated as hedges	Dec. 31, 2011 Not designated as hedges	Dec. 31, 2010 Not designated as hedges	Dec. 31, 2012 Not designated as hedges Natural gas swaps Regulatory assets	Dec. 31, 2012 Not designated as hedges Natural gas swaps Fuel	Dec. 31, 2011 Not designated as hedges Nuclear decommissioning trust Regulatory assets	Dec. 31, 2010 Not designated as hedges Nuclear decommissioning trust Regulatory assets	Dec. 31, 2012 Not designated as hedges Interest rate options Regulatory assets	Dec. 31, 2011 Not designated as hedges Interest rate options Regulatory assets
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains					$ 195						$ 2,338
Losses					(4,151)	(19,734)					(10,483)
Total losses on derivatives	(3,956)	(19,734)					(8,145)
Unrealized losses on derivatives			(7,220)	(2,054)						(1,085)		(2,557)	(2,564)	(74,217)	(30,554)
Unrealized gains on derivatives												3,602	2,689
Unrealized gains and (losses) on derivatives							$ (75,302)	$ (29,509)	$ 125