Stock-Based Compensation - Long-Term Stock Award (Details) - Long-term stock awards $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares
Unvested stock award shares
Balance at the beginning of the period (in shares) | shares	3,000,000	4,000,000
Granted (in shares) | shares	629,970	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	3,000,000	3,000,000
Weighted average grant date fair value
Balance at the beginning of the period (in dollars per share) | $ / shares	$ 24	$ 20
Granted (in dollars per share) | $ / shares	42	34
Vested (in dollars per share) | $ / shares	21	18
Forfeited (in dollars per share) | $ / shares	30	24
Balance at the end of the period (in dollars per share) | $ / shares	$ 30	$ 24
Additional disclosures
Total unrecognized compensation expense | $	$ 56	$ 56
Remaining weighted average vesting period	3 years	3 years
Total market value (at the vesting date) of stock award shares | $	$ 54	$ 41