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Stock Compensation Plans - Narrative (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value of stock options (in dollars per share)     $ 8.07 $ 9.80
Contractual term of stock options granted     7 years  
Unrecognized compensation cost related to unvested awards $ 28   $ 28  
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate period of U.S. Treasury yield     3 years  
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Risk-free interest rate period of U.S. Treasury yield     5 years  
ESPP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Discount stock purchase price percentage     15.00%  
Look-back feature period of discount stock purchase price     3 months  
Minimum contribution by participant percentage 1.00%   1.00%  
Maximum contribution by participant percentage 10.00%   10.00%  
Stocks purchased by employees during the period (in shares) 0.2 0.2    
Discounted price per share of stocks purchased by employees (in dollar per share) $ 26.44 $ 25.59    
Stock Option        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period of stock options grated     4 years  
Weighted average period to recognized compensation cost related to unvested awards     1 year 7 months 6 days  
Restricted Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unrecognized compensation cost related to unvested awards $ 101   $ 101  
Weighted average period to recognized compensation cost related to unvested awards     10 months 24 days