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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Fair Value Measurements [Line Items]          
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss) $ (683) $ (253) $ (1,100) $ (463)  
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) $ 380 $ 422 $ 1,200 $ 804  
Weighted Average [Member] | Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Inventory Related Payables [Member]          
Fair Value Measurements [Line Items]          
Basis 16.20%   16.20%   13.70%
Weighted Average [Member] | Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Inventories Carried At Market [Member]          
Fair Value Measurements [Line Items]          
Basis 30.20%   30.20%   4.30%