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Income Taxes Uncertain Tax Positions (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Unrecognized Tax Benefits [Roll Forward]        
Balance at beginning of period   $ 1,557 $ 1,182 $ 1,337
Additions to current year tax positions   292 160 49
Additions to prior years' tax positions   264 448 96
Reductions to prior years' tax positions   (244) (195) (192)
Reductions in tax positions due to lapse of statutory limitations   (38) (44) (103)
Settlements   (450) (11) (1)
Other   (40) 17 (4)
Balance at end of period $ 1,557 1,341 1,557 $ 1,182
Unrecognized tax benefit that would favorably affect effective tax rate 390 991 390  
Income tax related interest and penalties accrual 152 116 152  
Operating Loss Carryforwards [Line Items]        
Tax expense related to tax reform legislation $ 7,300   7,300  
Tax expense related to tax reform legislation, updated     $ 7,100  
Germany [Member] | Foreign Tax Authority [Member]        
Operating Loss Carryforwards [Line Items]        
Operating loss carryforwards   3,300    
Deferred tax assets   $ 1,000