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Stock Compensation Plans - Narrative (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 3 $ 6 $ 5 $ 12
Unearned compensation cost related to the unvested portion of all stock-based awards $ 39   $ 39  
Restricted Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Equity instruments granted (shares)     0.8  
Weighted average fair value per share granted (dollars per share)     $ 29.46  
Performance Share Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Equity instruments granted (shares)     0.2  
Weighted average fair value per share granted (dollars per share)     $ 29.04