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Share-Based Compensation (RSU Activity) (Details) - RSUs - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Period of straight-line expense, 2015 RSU's 3 years    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 10 months    
RSUs (Number of Shares):      
Nonvested at beginning of period 3,037    
Vested (1,337) (1,063) (954)
Forfeited and expired (280)    
Granted 1,546    
Nonvested at end of period 2,966 3,037  
Vested and expected to vest at end of period 2,711    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized $ 17    
Weighted Average Grant Date Fair Value (in dollars per share):      
Nonvested at beginning of period $ 10.70    
Vested 11.37    
Forfeited and expired 10.94    
Granted 11.93 $ 9.42 $ 12.03
Nonvested at end of period 11.02 $ 10.70  
Vested and expected to vest at end of period $ 11.01    
Nonvested at end of period, weighted average remaining vesting term 1 year 5 months