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 |