Share-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares Available for Grant
Options outstanding, beginning balance (in shares)	1,023,378	615,207
Additional shares reserved under plan (in shares)		1,200,000
Options granted (in shares)	(626,757)	(926,195)
Options forfeited (in shares)	302,755	134,366
Options outstanding, ending balance (in shares)	699,376	1,023,378
Shares Subject to Outstanding Options
Options outstanding, beginning balance (in shares)	1,399,484	825,130
Options granted (in shares)	727,257	926,195
Options forfeited (in shares)	(403,748)	(286,425)
Options exercised (in shares)	(51,327)	(65,416)
Options outstanding, ending balance (in shares)	1,671,666	1,399,484
Vested and expected to vest (in shares)	1,585,689	1,319,798
Exercisable (in shares)	1,007,870	678,480
Weighted-Average Exercise Price Per Share
Options outstanding, beginning balance (in USD per share)	$ 7.17	$ 11.27
Options granted (in USD per share)	2.97	5.15
Options forfeited (in USD per share)	7.61	13.81
Options exercised (in USD per share)	1.16	1.25
Options outstanding, ending balance (in USD per share)	5.42	7.17
Vested and expected (in USD per share)	5.53	7.23
Exercisable (in USD per share)	$ 6.58	$ 7.76
Weighted- Average Remaining Contractual Term (in years)
Options outstanding	8 years 2 months 1 day
Vested and expected	8 years 1 month 10 days	8 years 9 months 7 days
Exercisable	7 years 6 months 15 days	8 years 2 months 26 days
Aggregate Intrinsic Value
Options outstanding	$ 2
Vested and expected to vest	2	$ 390
Exercisable	$ 2	$ 366