Derivative instruments: (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended					12 Months Ended						12 Months Ended		12 Months Ended
	Dec. 31, 2013 Not designated as hedges	Dec. 31, 2012 Not designated as hedges	Dec. 31, 2011 Not designated as hedges	Dec. 31, 2012 Not designated as hedges Natural gas swaps Regulatory asset	Dec. 31, 2013 Not designated as hedges Natural gas swaps Regulatory liability	Dec. 31, 2013 Not designated as hedges Natural gas swaps Fuel	Dec. 31, 2012 Not designated as hedges Natural gas swaps Fuel	Dec. 31, 2013 Not designated as hedges Interest rate options Regulatory asset	Dec. 31, 2012 Not designated as hedges Interest rate options Regulatory asset	Dec. 31, 2011 Not designated as hedges Interest rate options Regulatory asset	Dec. 31, 2011 Not designated as hedges Nuclear decommissioning trust Regulatory asset	Dec. 31, 2011 Designated as hedges	Dec. 31, 2011 Designated as hedges Natural gas swaps Accounts receivable	Dec. 31, 2011 Designated as hedges Natural gas swaps Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains						$ 739	$ 2,338							$ 195
Losses						(4,051)	(10,483)							(4,151)
Total gains (losses) on derivatives	(3,312)	(8,145)										(3,956)
Unrealized losses on derivatives				(1,085)				(36,529)	(74,217)	(30,554)	(2,557)		(7,220)
Unrealized gains on derivatives					1,011						3,602
Unrealized gains and (losses) on derivatives	$ (35,518)	$ (75,302)	$ (29,509)