Stock Option and Incentive Plans - Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Stock option
Stock option and incentive plans
Total compensation cost	$ 752	$ 597	$ 2,228	$ 1,368
Stock Options
Outstanding (in shares)	2,238,197		2,238,197
Outstanding, weighted-average exercise price (in dollars per share)	$ 6.60		$ 6.60
Granted (in shares)				596,475
Granted, weighted-average exercise price (in dollars per share)				$ 2.44
Exercised (in shares)				118,365
Exercised, weighted-average exercise price (in dollars per share)				$ 1.04
Forfeited (in shares)				39,139
Forfeited, weighted-average exercise price (in dollars per share)				$ 12.42
Stock option | Research and development
Stock option and incentive plans
Total compensation cost	$ 265	179	$ 710	$ 436
Stock option | General and administrative
Stock option and incentive plans
Total compensation cost	487	$ 418	1,518	$ 932
Nonvested stock
Stock option and incentive plans
Total compensation cost	48		48
Nonvested Stock
Unrecognized compensation cost	$ 200		$ 200
Weighted-average recognition period of unrecognized compensation cost			10 months 2 days
Vested (in shares)	0		0
Nonvested stock | Research and development
Stock option and incentive plans
Total compensation cost	$ 19		$ 19
Nonvested stock | General and administrative
Stock option and incentive plans
Total compensation cost	$ 29		$ 29
2005 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	0		0
2005 Plan | Stock option
Stock Options
Outstanding (in shares)	430,922		430,922
2014 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	22,023		22,023
2014 Plan | Stock option
Stock Options
Outstanding (in shares)	1,807,275		1,807,275
2014 Plan | Nonvested stock
Nonvested Stock
Outstanding (in shares)	196,275		196,275
Issued (in shares)	196,275		196,275