Employee Benefit Plans - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Number of Shares
Outstanding, vested and expected to vest at December 31, 2018	18,452,375
Granted	2,225,766
Exercised	(1,327,784)
Forfeited	(447,402)
Expired	(886,650)
Outstanding, vested and expected to vest at March 31, 2019	18,016,305	18,452,375
Vested and exercisable at March 31, 2019	9,234,768
Weighted Average Exercise Price
Outstanding, vested and expected to vest at December 31, 2018	$ 14.39
Granted	12.35
Exercised	10.86
Forfeited	14.33
Expired	15.99
Outstanding, vested and expected to vest at March 31, 2019	14.32	$ 14.39
Vested and exercisable at March 31, 2019	$ 14.72
Additional Disclosures
Outstanding, vested and expected to vest, Aggregate Intrinsic Value, Beginning balance	$ 70,104	$ 8,482
Vested and exercisable at March 31, 2019	$ 32,262
Outstanding, vested and expected to vest, Weighted Average Remaining Contractual Term	7 years 8 months 12 days	7 years 6 months
Vested and exercisable, Weighted Average Remaining Contractual Term	6 years 8 months 12 days