Derivative Instruments - Interest rate options (Details) - Interest rate options	3 Months Ended	6 Months Ended
	Dec. 31, 2011 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative instruments and hedging activities
Purchased amount of derivative instrument	$ 100,000,000
Notional Dollar Amount	$ 2,200,000,000	$ 238,034,000
Number of nuclear units expected to be financed | item	2
Derivative notional amount expired		1,941,169,000
Derivative remaining notional amount		238,034,000
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)		3.88%
Notional amount of derivatives, expired without value		$ 152,668,000
Fair value of assets		5,000	$ 1,010,000
Funds posted as collateral of the counterparties		0	$ 0
2016
Derivative instruments and hedging activities
Notional Dollar Amount		157,865,000
2017
Derivative instruments and hedging activities
Notional Dollar Amount		$ 80,169,000
LIBOR
Derivative instruments and hedging activities
Variable rate basis		LIBOR
Minimum
Derivative instruments and hedging activities
Collateral thresholds range		$ 0
Maximum
Derivative instruments and hedging activities
Collateral thresholds range		$ 10,000,000
Average | LIBOR
Derivative instruments and hedging activities
Basis spread (as a percent)		2.24%
Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt		$ 2,200,000,000