Derivative instruments (Details 2) (Interest rate options, USD $)	3 Months Ended	9 Months Ended
	Dec. 31, 2011 item	Sep. 30, 2014	Dec. 31, 2013
Derivative instruments and hedging activities
Purchased amount of derivative instrument	$ 100,000,000
Variable rate basis		LIBOR
Notional Dollar Amount	2,200,000,000	1,004,955,000
Number of nuclear units expected to be financed	2
Derivative notional amount expired		1,174,248,000
Derivative remaining notional amount		1,004,955,000
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)		4.08%
Notional amount of derivatives, expired without value		419,796,000
Additional premium payment obligations on interest rate option transactions		0
Fair value of assets		13,110,000	63,471,000
Funds posted as collateral by the counterparties		0	34,970,000
Carrying amount of derivative assets		13,110,000	28,501,000
2014
Derivative instruments and hedging activities
Notional Dollar Amount		143,629,000
2015
Derivative instruments and hedging activities
Notional Dollar Amount		470,624,000
2016
Derivative instruments and hedging activities
Notional Dollar Amount		310,533,000
2017
Derivative instruments and hedging activities
Notional Dollar Amount		80,169,000
Minimum
Derivative instruments and hedging activities
Collateral thresholds range		0
Maximum
Derivative instruments and hedging activities
Collateral thresholds range		$ 10,000,000
Average
Derivative instruments and hedging activities
Fixed interest rates above current LIBOR swap rates (as a percent)		1.07%