Stockholder's Equity - Shares Available for Future Grant and Issuance (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Shares Reserved for Future Issuance
Stock options outstanding (in shares)			10,397,243	8,624,223
Employee stock purchase plan (in shares)			225,806
Warrants outstanding (in shares)			123,091
Total shares of common stock reserved for issuance (in shares)			12,662,116
2013 plan
Shares Available for Future Grant
Balance at the beginning of the period (in shares)	991,613
Authorized (in shares)	2,492,431
Granted (in shares)	(2,505,625)
Forfeitures/Cancelled (in shares)	687,057
Balance at the end of the period (in shares)	1,665,476
Shares Reserved for Future Issuance
Stock options outstanding (in shares)			10,147,743
Shares available for future grant (in shares)	991,613		1,665,476	991,613
Inducement Plan
Shares Available for Future Grant
Balance at the beginning of the period (in shares)	293,000
Granted (in shares)	(95,000)
Forfeitures/Cancelled (in shares)	52,500
Balance at the end of the period (in shares)	250,500
Shares Reserved for Future Issuance
Stock options outstanding (in shares)			249,500
Shares available for future grant (in shares)	293,000		250,500	293,000
Employee Stock Option | Weighted-average
Weighted-average assumptions:
Expected term of options (in years)	5 years 9 months 18 days	6 years 3 months 18 days
Risk-free interest rate (as a percent)	2.70%	2.10%
Expected volatility (as a percent)	76.00%	75.70%
Dividend yield (as a percent)	0.00%	0.00%