Derivative Instruments (Details 2) (USD $)	3 Months Ended	6 Months Ended
	Dec. 31, 2011	Jun. 30, 2013	Dec. 31, 2012
Derivative instruments and hedging activities
Variable rate basis		LIBOR
Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Number of nuclear units expected to be financed	2
LIBOR swaptions
Derivative instruments and hedging activities
Purchased amount of derivative instrument	$ 100,000,000
Variable rate basis		LIBOR
Cash settlement value, if swap rates are at or below the specified fixed rate on the expiration date		0
Weighted average fixed rate (as a percent)		4.15%
Notional amount of derivatives, expired without value		373,396,000
Additional premium payment obligations on interest rate option transactions		0
Fair value of assets		43,680,000	25,783,000
Funds posted as collateral by the counterparties		36,070,000	8,950,000
Carrying amount of derivative assets		7,610,000	16,833,000
Notional Dollar Amount		1,805,808,000
LIBOR swaptions | 2013
Derivative instruments and hedging activities
Notional Dollar Amount		381,056,000
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)		0.50%
Collateral thresholds range		0
LIBOR swaptions | Maximum
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)		1.50%
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	$ 1,800,000,000