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Derivative and Financial Instruments - Commodity Notional (Details) - Commodity Contract [Member] - Bcfe
Bcfe in Billions
Dec. 31, 2018
Dec. 31, 2017
Natural gas and power, Fixed price [Member] | Short [Member]    
Trading Activity, Gains and Losses, Net [Line Items]    
Commodity derivatives - volumetric material net exposures (17) (29)
Natural gas and power, Basis [Member] | Long [Member]    
Trading Activity, Gains and Losses, Net [Line Items]    
Commodity derivatives - volumetric material net exposures   (12)
Natural gas and power, Basis [Member] | Short [Member]    
Trading Activity, Gains and Losses, Net [Line Items]    
Commodity derivatives - volumetric material net exposures (1)