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Stock-Based Compensation - Summary (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense $ 13 $ 19
Stock option expense    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense 1 1
Restricted stock and unit awards expense    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense 12 $ 18
Unrecognized compensation expense $ 53  
Unrecognized compensation expense, period for recognition 1 year 6 months