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, 2021	Mar. 31, 2020
Unvested stock award shares
Balance at the beginning of the period (in shares)	1	2
Vested (in shares)	0	1
Forfeited (in shares)	0	0
Balance at the end of the period (in shares)	1	1
Weighted average grant date fair value
Balance at the beginning of the period (in dollars per share)	$ 36	$ 34
Vested (in dollars per share)	34	32
Forfeited (in dollars per share)	37	35
Balance at the end of the period (in dollars per share)	$ 37	$ 36
Additional disclosures
Total unrecognized compensation expense	$ 18	$ 32
Weighted average remaining vesting period	2 years	3 years
Total market value (at the vesting date) of stock award shares	$ 26	$ 29