Stockholders' Equity - Schedule of Pre-2017 Non-Qualified Stock Options Outstanding (Details) - Pre-2017 Non-Qualified Stock Options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Shares
Outstanding at beginning of period (in shares)	2,885,500
Exercised (in shares)	(80,000)
Forfeited (in shares)	(4,865)
Outstanding at end of period (in shares)	2,800,635	2,885,500
Exercisable (in shares)	2,695,766
Weighted- average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.30
Exercised (in dollars per share)	7.52
Forfeited (in dollars per share)	10.00
Outstanding at end of period (in dollars per share)	7.29	$ 7.30
Exercisable (in dollars per share)	$ 7.21
Weighted- average Remaining Contractual Term (years)
Outstanding	5 years 9 months 7 days	6 years 15 days
Exercisable	5 years 9 months 4 days
Aggregate Intrinsic Value (in Thousands)
Outstanding	$ 16,358,456	$ 20,633,100
Exercisable	$ 15,932,550