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Income Taxes (Schedule of Unrecognized 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 [Roll Forward]      
Balance, January 1 $ 102 $ 143 $ 141
Additions for tax positions of the current year 25 14 12
Additions for tax positions of prior years 110 1 27
Reductions for tax positions of prior years (1) (33) (25)
Settlements during the period (10) (20) (5)
Positions assumed in acquisitions 0 0 0
Lapses of applicable statute of limitation (7) (3) (7)
Balance, December 31 $ 219 $ 102 $ 143