Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Number of Options
Outstanding, beginning balance (in shares)	13,590
Granted (in shares)	1,064
Exercised (in shares)	(1,139)
Cancelled (in shares)	(17)
Outstanding, ending balance (in shares)	13,498
Options vested and exercisable (in shares)	8,154
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.07
Granted (in dollars per share)	14.57
Exercised (in dollars per share)	3.86
Cancelled (in dollars per share)	10.30
Outstanding, ending balance (in dollars per share)	6.92
Options vested and exercisable (in dollars per share)	$ 6.21
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual life, options outstanding	6 years 8 months 15 days	6 years 7 months 17 days
Weighted-average remaining contractual life, options vested and exercisable	5 years 8 months 1 day
Aggregate intrinsic value, options outstanding	$ 97,127		$ 66,462
Aggregate intrinsic value, options vested and exercisable	$ 64,261