Stockholders' Equity - Schedule of Pre-2017 Non-Qualified Stock Options Outstanding (Details) - Pre-2017 Non-Qualified Stock Options $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares
Outstanding at beginning of period (in shares) | shares	2,980,500
Exercised (in shares) | shares	(15,000)
Forfeited (in shares) | shares	0
Outstanding at end of period (in shares) | shares	2,965,500	2,980,500
Exercisable (in shares) | shares	2,803,399
Weighted- average Exercise Price
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 7.33
Exercised (in dollars per share) | $ / shares	10.00
Forfeited (in dollars per share) | $ / shares	0.00
Outstanding at end of period (in dollars per share) | $ / shares	7.32	$ 7.33
Exercisable (in dollars per share) | $ / shares	$ 7.20
Weighted- average Remaining Contractual Term (years)
Outstanding	6 years 3 months 29 days	7 years 22 days
Exercisable	6 years 2 months 9 days
Aggregate Intrinsic Value (in Thousands)
Outstanding | $	$ 31,258	$ 46,606
Exercisable | $	$ 29,889