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Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details)
MWh in Thousands, MMBTU in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2015
MWh
MMBTU
Dec. 31, 2014
MWh
MMBTU
Not Designated as Hedging Instrument | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume 10,131 9,690
Not Designated as Hedging Instrument | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume 993 2,710
Not Designated as Hedging Instrument | Electricity    
Derivatives, Fair Value [Line Items]    
Net notional volume | MWh 1,368 607
Designated as Hedging Instrument | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume (160) 155
Designated as Hedging Instrument | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume 760 56