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Income Taxes - Uncertain Tax Positions (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Unrecognized Tax Benefits [Roll Forward]      
Balance at beginning of period $ 1,341 $ 1,557 $ 1,182
Additions to current year tax positions 18 292 160
Additions to prior years' tax positions 13 264 448
Reductions to prior years' tax positions (501) (244) (195)
Reductions in tax positions due to lapse of statutory limitations (8) (38) (44)
Settlements (93) (450) (11)
Other 5 (40) 17
Balance at end of period 775 1,341 1,557
Unrecognized tax benefit that would favorably affect effective tax rate 539 991  
Income tax related interest and penalties accrual 117 116  
Operating Loss Carryforwards [Line Items]      
Tax expense related to tax reform legislation     7,300
Tax expense related to tax reform legislation, updated     $ 7,100
Germany | Foreign Tax Authority      
Operating Loss Carryforwards [Line Items]      
Operating loss carryforwards $ 3,300    
Deferred tax assets   $ 1,000