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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, 2019
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2019
Jun. 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 7 years  
Unrecognized compensation cost related to unvested awards $ 29     $ 29  
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 0.1      
Discounted price per share of stocks purchased by employees (in dollar per share) $ 24.02 $ 26.18      
Stock Option          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of stock options grated     4 years 4 years  
Weighted average period to recognized compensation cost related to unvested awards       1 year 8 months 12 days  
Restricted Stock Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized compensation cost related to unvested awards $ 119     $ 119  
Weighted average period to recognized compensation cost related to unvested awards       1 year