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Derivative and Financial Instruments - Commodity Notional (Details) - Short [Member] - Commodity Contract [Member] - Bcfe
Jun. 30, 2019
Dec. 31, 2018
Natural gas and power, Fixed price [Member]    
Trading Activity, Gains and Losses, Net [Line Items]    
Commodity derivatives - volumetric material net exposures 19 17
Natural gas and power, Basis [Member]    
Trading Activity, Gains and Losses, Net [Line Items]    
Commodity derivatives - volumetric material net exposures 12 1