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Derivative and hedging instruments Part 2 (Details) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
E.ON Contingent Obligation - Net [Member] \| Net Gain (Loss) on Forward and Derivative Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income from derivatives	$ 353	$ 353
E.ON Contingent Obligation - Net [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income from derivatives	(353)	(353)
Midwest Premium Contracts [Member] \| Net Gain (Loss) on Forward and Derivative Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income from derivatives	$ 0	$ (1,080)