Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Number of Options
Number of stock options, outstanding, balance (in shares)	16,268
Number of stock options, granted (in shares)	1,436
Number stock options, exercised (in shares)	(1,529)
Number of stock options, cancelled (in shares)	(480)
Number of stock options, outstanding, balance (in shares)	15,695	16,268
Weighted Average Exercise Price
Weighted-average exercise price, outstanding, balance (in dollars per share)	$ 5.70
Weighted-average exercise price, granted (in dollars per share)	8.33
Weighted-average exercise price, exercised (in dollars per share)	5.25
Weighted-average exercise price, cancelled (in dollars per share)	4.33
Weighted-average exercise price, outstanding, balance (in dollars per share)	$ 5.94	$ 5.70
Weighted-average remaining contractual life, options outstanding	7 years 3 months 7 days	7 years 4 months 28 days
Aggregate intrinsic value, options outstanding	$ 114	$ 14,832
Options vested and exercisable, number of options (in shares)	8,624
Options vested and exercisable, weighted average exercise price (in dollars per share)	$ 5.52
Options vested and exercisable, weighted average remaining contractual life	6 years 2 months 4 days
Options vested and exercisable, aggregate intrinsic value	$ 66,074