Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details)	12 Months Ended
	Dec. 31, 2014 MMBTU	Dec. 31, 2013 MMBTU
Not Designated as Hedging Instrument | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume	9,690,000	3,513,000
Not Designated as Hedging Instrument | Natural Gas Basis
Derivatives, Fair Value [Line Items]
Net notional volume	2,710,000	373,000
Not Designated as Hedging Instrument | Electricity
Derivatives, Fair Value [Line Items]
Net notional volume	607,000	465,000
Designated as Hedging Instrument | Natural Gas
Derivatives, Fair Value [Line Items]
Net notional volume	(155,000)	2,259,000
Designated as Hedging Instrument | Natural Gas Basis
Derivatives, Fair Value [Line Items]
Net notional volume	(56,000)	1,443,000