Share-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	18,096	16,285	8,593
Granted (in shares)	1,000	3,611	9,226
Exercised (in shares)	0	0	0
Expired (in shares)	(553)	(602)	(435)
Forfeited (in shares)	(609)	(1,198)	(1,099)
Outstanding, ending balance (in shares)	17,934	18,096	16,285	8,593
Options exercisable, shares underlying options (in shares)	13,092	8,250	4,474
Weighted Average Exercise Price Per Share
Outstanding, beginning balance (in usd per share)	$ 7.20	$ 8.25	$ 11.88
Granted (in usd per share)	2.97	3.01	5.45
Exercised (in usd per share)	0	0	0
Expired (in usd per share)	6.36	13.83	18.50
Forfeited (in usd per share)	3.97	5.45	9.12
Outstanding, ending balance (in usd per share)	7.10	7.20	8.25	$ 11.88
Options exercisable, average exercise price per share (in usd per share)	$ 8.28	$ 10.73	$ 15.15
Options outstanding, weighted average contract life	5 years 8 months 12 days	7 years 1 month 24 days	7 years 8 months 23 days	7 years 2 months 19 days
Options outstanding, aggregate intrinsic value	$ 0	$ 0	$ 1	$ 14
Options exercised, aggregate intrinsic value	$ 0	$ 0	$ 0
Options exercisable, weighted average contract life	4 years 10 months 9 days	5 years 8 months 23 days	5 years 3 months 3 days
Options exercisable, aggregate intrinsic value	$ 0	$ 0	$ 0