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Accounting for Derivative Instruments - Volumetric Underlying Derivative Transactions (Details)
Dec. 31, 2016
MWh
MMBTU
Dec. 31, 2015
MWh
MMBTU
Non-trading | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume buy (sell) 8,016 7,543
Non-trading | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume buy (sell) 0 455
Non-trading | Electricity    
Derivatives, Fair Value [Line Items]    
Net notional volume buy (sell) | MWh 3,958 1,187
Trading | Natural Gas    
Derivatives, Fair Value [Line Items]    
Net notional volume buy (sell) (953) 8
Trading | Natural Gas Basis    
Derivatives, Fair Value [Line Items]    
Net notional volume buy (sell) (380) (455)