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Income Taxes Uncertain Tax Positions (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unrecognized Tax Benefits [Roll Forward]        
Beginning balance   $ 1,182 $ 1,337 $ 1,705
Additions to current year tax positions   160 49 53
Additions to prior years' tax positions   448 96 114
Reductions to prior years' tax positions   (195) (192) (349)
Reductions in tax positions due to lapse of statutory limitations   (44) (103) (119)
Settlements   (11) (1) (3)
Other   17 (4) (64)
Ending balance $ 1,557 1,557 1,182 $ 1,337
Unrecognized tax benefit that would favorably affect effective tax rate 390 390 682  
Income tax related interest and penalties accrual 152 152 $ 160  
Other Matters        
Tax expense related to tax reform legislation $ 7,300 $ 7,300