Share-Based Compensation - Stock Option Activity Table (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares options)	18,096
Granted (in shares options)	1,000
Exercised (in shares options)	0
Expired (in shares options)	(87)
Forfeited (in shares options)	(79)
Outstanding, ending balance (in shares options)	18,930	18,096
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average exercise price, beginning (in dollars per share)	$ 7.20
Weighted average exercise price, granted (in dollars per share)	2.97
Weighted average exercise price, exercised (in dollars per share)	0.00
Weighted average exercise price, expired (in dollars per share)	11.23
Weighted average exercise price, forfeited (in dollars per share)	5.45
Weighted average exercise price, ending (in dollars per share)	$ 6.97	$ 7.20
Weighted average contract life, outstanding (in years)	7 years 10 days	7 years 1 month 24 days
Aggregate intrinsic value, outstanding, beginning balance	$ 0
Aggregate intrinsic value, exercised	0
Aggregate intrinsic value, outstanding, ending balance	$ 0	$ 0
Exercisable (in shares options)	13,355
Weighted average exercise price, exercisable (in dollars per share)	$ 8.23
Weighted average contract life, exercisable (in years)	6 years 3 months 29 days
Aggregate intrinsic value, exercisable	$ 0