Share-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Shares
Beginning balance (in shares)	76,635
Exercised (in shares)	(73)
Forfeited (in shares)	(6)
Ending balance (in shares)	76,556	76,635
Exercisable (in shares)	76,351
Weighted-Average Exercise Price
Beginning balance (in dollars per share)	$ 2.22
Exercised (in dollars per share)	2.58
Forfeited (in dollars per share)	3.84
Ending balance (in dollars per share)	2.22	$ 2.22
Exercisable (in dollars per share)	$ 2.21
Weighted-Average Remaining Contractual Term, Outstanding	4 years 3 months 18 days	4 years 6 months
Weighted-Average Remaining Contractual Term, Exercisable	4 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding	$ 1,339,176	$ 1,285,338
Aggregate Intrinsic Value, Exercisable	$ 1,335,964