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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Fair Value Measurements [Line Items]          
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss) $ (253) $ (157) $ (463) $ (177)  
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) $ 422 $ 280 $ 804 $ 491  
Weighted Average [Member] | Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Inventory Related Payables [Member]          
Fair Value Measurements [Line Items]          
Basis 13.50%   13.50%   14.70%
Weighted Average [Member] | Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Inventories Carried At Market [Member]          
Fair Value Measurements [Line Items]          
Basis 17.50%   17.50%   28.20%