Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Number of Options
Number of stock options, outstanding, balance (in shares)	12,897,000
Number of stock options, granted (in shares)	4,585,000
Number stock options, exercised (in shares)	(207,000)
Number of stock options, cancelled (in shares)	(524,000)
Number of stock options, outstanding, balance (in shares)	16,751,000	12,897,000
Number of stock options, options vested and exercisable (in shares)	7,961,000
Weighted Average Exercise Price
Weighted-average exercise price, outstanding, balance (in dollars per share)	$ 5.94
Weighted-average exercise price, granted (in dollars per share)	5.03
Weighted-average exercise price, exercised (in dollars per share)	2.49
Weighted-average exercise price, cancelled (in dollars per share)	7.82
Weighted-average exercise price, outstanding, balance (in dollars per share)	5.68	$ 5.94
Weighted-average exercise price, options vested and exercisable (in dollars per share)	$ 5.40
Weighted-average remaining contractual life, options outstanding	7 years 9 months	7 years 5 months 26 days
Weighted-average remaining contractual life, options vested and exercisable	6 years 4 months 2 days
Aggregate intrinsic value, options outstanding	$ 32,028
Aggregate intrinsic value, options vested and exercisable	$ 18,378