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Income Taxes (Schedule of Unrecognized Tax Benefits Roll Forward) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of gross amounts of unrecognized gross tax benefits    
Amount of unrecognized tax benefits, beginning balance $ 87 $ 98
Amount of decreases due to lapse of the applicable statute of limitations (12) (4)
Amount of decreases due to settlements (19) (23)
Increases as a result of tax positions taken in the current period 5 2
Increases as a result of tax positions taken in a prior period 14 14
Amount of unrecognized tax benefits, ending balance $ 75 $ 87