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Stock-Based Compensation - Long-Term Stock Award (Details) - Long-term stock awards - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Unvested stock award shares    
Balance at the beginning of the period (in shares) 2 2
Granted (in shares) 0 1
Vested (in shares) 1 1
Forfeited (in shares) 0 0
Balance at the end of the period (in shares) 1 2
Weighted average grant date fair value    
Balance at the beginning of the period (in dollars per share) $ 34 $ 30
Granted (in dollars per share) 0 36
Vested (in dollars per share) 32 24
Forfeited (in dollars per share) 35 29
Balance at the end of the period (in dollars per share) $ 36 $ 34
Additional disclosures    
Total unrecognized compensation expense $ 32 $ 61
Weighted average remaining vesting period 3 years 4 years
Total market value (at the vesting date) of stock award shares $ 29 $ 29