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Income Taxes (Unrecognized Income Tax Benefits) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns      
Balance at January 1 $ 321 $ 406 $ 416
Gross increases for tax positions of prior years 50 20 80
Gross decreases for tax positions of prior years (23) (104) (61)
Gross increases for tax positions of the current year 37 39 59
Settlements (19) (29) (63)
Other (12) (11) (25)
Balance at December 31 $ 354 $ 321 $ 406