Derivative Instruments - Interest rate options (Details) - Interest rate options	3 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2011 USD ($) item	Dec. 31, 2015 USD ($)
Derivative instruments and hedging activities
Purchased amount of derivative instrument		$ 100,000,000
Notional Dollar Amount	$ 314,810,000	$ 2,200,000,000
Number of nuclear units expected to be financed | item		2
Derivative notional amount expired	1,864,393,000
Derivative remaining notional amount	314,810,000
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)	3.93%
Notional amount of derivatives, expired without value	$ 75,892,000
Fair value of assets	95,000		$ 1,010,000
Funds posted as collateral of the counterparties	0		$ 0
2016
Derivative instruments and hedging activities
Notional Dollar Amount	234,641,000
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)	1.98%
Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt	$ 2,200,000,000