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Accounting for Derivative Instruments and Hedging Activities - Fair Value Within the Derivative Instrument Valuation (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Derivative [Line Items]    
Derivative assets $ 1,170 $ 1,081
Commodity contracts 1,103 977
Interest rate contracts current    
Derivative [Line Items]    
Derivative assets 0 17
Commodity contracts 0 0
Interest rate contracts long-term    
Derivative [Line Items]    
Derivative assets 0 22
Commodity contracts 0 0
Commodity contracts current    
Derivative [Line Items]    
Derivative assets 860 747
Commodity contracts 781 673
Commodity contracts long-term    
Derivative [Line Items]    
Derivative assets 310 295
Commodity contracts $ 322 $ 304