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Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details)
MWh in Thousands, MMBTU in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2015
MWh
MMBTU
Dec. 31, 2014
MWh
MMBTU
Not Designated as Hedging Instrument | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume 6,677 9,690
Not Designated as Hedging Instrument | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume 1,303 2,710
Not Designated as Hedging Instrument | Electricity    
Derivatives, Fair Value [Line Items]    
Net notional volume | MWh 789 607
Designated as Hedging Instrument | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume 627 155
Designated as Hedging Instrument | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume 78 56