Equity and Employee Stock Plans - Schedule of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ 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)	9,435,349
Options exercised (in shares)	(679,495)
Options forfeited (in shares)	(40,550)
Options canceled (in shares)	(2,142)
Outstanding, ending balance (in shares)	8,713,162
Options exercisable at period end (in shares)	7,258,130
Weighted- Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.48
Options exercised (in dollars per share)	5.49
Options forfeited (in dollars per share)	9.15
Options canceled (in dollars per share)	8.35
Outstanding, ending balance (in dollars per share)	6.55
Options exercisable at period end (in dollars per share)	$ 6.01
Weighted-Average Remaining Contractual Life (years)
Weighted average remaining contractual life outstanding	6 years	6 years 22 days
Weighted average remaining contractual life exercisable	5 years 8 months 9 days
Aggregate Intrinsic Value
Options outstanding, Aggregate intrinsic value	$ 119,575		$ 74,669
Options exercisable, Aggregate intrinsic value	$ 103,486