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Stock-Based Compensation - Long-Term Stock Award (Details) - Long-term stock awards
$ / shares in Units, $ in Millions
 9 Months Ended
Sep. 30, 2019
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
shares
Unvested stock award shares    
Balance at the beginning of the period (in shares) | shares 2,000,000 3,000,000
Granted (in shares) | shares 636,030 1,000,000
Vested (in shares) | shares 1,000,000 2,000,000
Forfeited (in shares) | shares 0 0
Balance at the end of the period (in shares) | shares 2,000,000 2,000,000
Weighted average grant date fair value    
Balance at the beginning of the period (in dollars per share) | $ / shares $ 30 $ 24
Granted (in dollars per share) | $ / shares 36 42
Vested (in dollars per share) | $ / shares 25 21
Forfeited (in dollars per share) | $ / shares 32 31
Balance at the end of the period (in dollars per share) | $ / shares $ 34 $ 30
Additional disclosures    
Total unrecognized compensation expense | $ $ 49 $ 50
Remaining weighted average vesting period 3 years 3 years
Total market value (at the vesting date) of stock award shares | $ $ 31 $ 56