Derivative instruments: (Details 2) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012
Derivative instruments and hedging activities
Variable rate basis		LIBOR
LIBOR swaptions
Derivative instruments and hedging activities
Purchased amount of derivative instrument	$ 100,000,000
Variable rate basis		LIBOR
Weighted average fixed rate (as a percent)		4.17%
Fair value of assets	69,446,000	25,783,000
Funds posted as collateral by the counterparties	43,070,000	8,950,000
Carrying amount of derivative assets	26,376,000	16,833,000
Notional Dollar Amount		2,179,204,000
LIBOR swaptions | 2013
Derivative instruments and hedging activities
Notional Dollar Amount		754,452,000
LIBOR swaptions | 2014
Derivative instruments and hedging activities
Notional Dollar Amount		563,425,000
LIBOR swaptions | 2015
Derivative instruments and hedging activities
Notional Dollar Amount		470,625,000
LIBOR swaptions | 2016
Derivative instruments and hedging activities
Notional Dollar Amount		310,533,000
LIBOR swaptions | 2017
Derivative instruments and hedging activities
Notional Dollar Amount		80,169,000
LIBOR swaptions | Minimum
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)		1.50%
Collateral thresholds range		0
LIBOR swaptions | Maximum
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)		2.00%
Collateral thresholds range		10,000,000
LIBOR swaptions | Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt	$ 2,200,000,000