Stock-Based Compensation - Long-Term Stock Award (Details) - Long-term stock awards - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 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	25
Forfeited (in dollars per share)	35	32
Balance at the end of the period (in dollars per share)	$ 36	$ 34
Additional disclosures
Total unrecognized compensation expense	$ 25	$ 49
Weighted average remaining vesting period	2 years	3 years
Total market value (at the vesting date) of stock award shares	$ 31	$ 31