Stockholder's Equity - Shares Available for Future Issuance (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercisable			$ 2.5
Weighted average exercise price (in dollars per share)			$ 5.18
Shares Reserved for Future Issuance
Stock options outstanding (in shares)			8,461,435	6,370,578
Employee stock purchase plan			225,806
Warrants outstanding (in shares)			123,091
Total shares of common stock reserved for issuance			10,267,547
2013 plan
Shares Available for Future Grant
Balance at the beginning of the period (in shares)	1,101,331
Authorized (in shares)			2,230,736
Granted (in shares)	(2,507,694)
Forfeitures/Cancelled (in shares)	132,842
Balance at the end of the period (in shares)	957,215
Shares Reserved for Future Issuance
Shares available for future grant	1,101,331		957,215	1,101,331
Inducement Plan
Shares Available for Future Grant
Balance at the end of the period (in shares)	500,000
Shares Reserved for Future Issuance
Shares available for future grant	500,000		500,000
Options outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share price of Company's closing stock price (in dollars per share)			$ 3.67
Per-share weighted-average grant date fair value of options granted (in dollars per share)	$ 3.35	$ 5.47
Weighted-average assumptions:
Unrecognized compensation expense			$ 19.6
Weighted average remaining period for recognition of unrecognized compensation expense	3 years 26 days
Options outstanding | Weighted-average
Weighted-average assumptions:
Expected term of options (in years)	6 years 3 months 18 days	6 years 3 months 18 days
Risk-free interest rate (as a percent)	2.10%	1.50%
Expected volatility (as a percent)	75.70%	67.80%
Dividend yield (as a percent)	0.00%	0.00%
Estimated annual forfeiture rate (as a percent)	0.00%