Stockholders' Equity - Summary of Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, Beginning balance (in shares)	3,590,393	2,832,467
Options granted (in shares)	954,301	991,110
Options exercised (in shares)	(175,183)	(20,782)
Options forfeited (in shares)	(117,124)	(182,605)
Options expired (in shares)	(157,855)	(29,797)
Options outstanding, Ending balance (in shares)	4,094,532	3,590,393	2,832,467
Options vested and expected to vest (in shares)	4,094,532
Exercisable options (in shares)	2,464,086
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Beginning balance (in dollars per share)	$ 12.13	$ 11.48
Options granted (in dollars per share)	6.46	15.49
Options exercised (in dollars per share)	2.56	3.12
Options forfeited (in dollars per share)	14.35	19.01
Options expired (in dollars per share)	17.19	25.65
Ending balance (in dollars per share)	10.96	$ 12.13	$ 11.48
Options vested and expected to vest (in dollars per share)	10.96
Exercisable options (in dollars per share)	$ 10.98
Weighted average remaining contract term for outstanding options	7 years 2 months 15 days	7 years 3 months 29 days
Weighted average remaining contract term for options vested and expected to vest	7 years 2 months 15 days
Weighted average remaining contract term for exercisable options	6 years 3 months 14 days
Aggregate intrinsic value, outstanding	$ 2,771	$ 5,062
Aggregate intrinsic value of options vested and expected to vest	2,771
Aggregate intrinsic value of exercisable options	$ 2,771