Stock-Based Compensation - Long-Term Stock Award (Details) - Long-term stock awards - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2017
Unvested stock award shares
Balance at the beginning of the period (in shares)	2,000,000	3,000,000
Granted (in shares)	632,280	1,000,000
Vested (in shares)	1,000,000	1,000,000
Forfeited (in shares)	0	0
Balance at the end of the period (in shares)	2,000,000	3,000,000
Weighted average grant date fair value
Balance at the beginning of the period (in dollars per share)	$ 30	$ 24
Granted (in dollars per share)	36	42
Vested (in dollars per share)	25	21
Forfeited (in dollars per share)	31	30
Balance at the end of the period (in dollars per share)	$ 34	$ 30
Additional disclosures
Total unrecognized compensation expense	$ 55		$ 56
Remaining weighted average vesting period	3 years	3 years
Total market value (at the vesting date) of stock award shares	$ 30	$ 54