Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock options
Stock option and incentive plans
Total compensation cost	$ 1,153	$ 553	$ 3,576	$ 1,878
Stock Options
Outstanding (in shares)	4,832,109		4,832,109
Outstanding, weighted-average exercise price (in dollars per share)	$ 5.54		$ 5.54
Granted (in shares)			1,212,000
Granted (in dollars per share)			$ 6.45
Stock options | Research and development
Stock option and incentive plans
Total compensation cost	$ 458	172	$ 1,274	542
Stock options | General and administrative
Stock option and incentive plans
Total compensation cost	695	381	2,302	1,336
Restricted stock
Stock option and incentive plans
Total compensation cost	$ 18	179	$ 69	$ 373
Shares
Outstanding (in shares)	117,867		117,867
Issued (in shares)			0	245,200
Vested (in shares)			113,933
Restricted stock | Minimum
Stock option and incentive plans
Vesting period			1 year
Restricted stock | Maximum
Stock option and incentive plans
Vesting period			3 years
Restricted stock | Research and development
Stock option and incentive plans
Total compensation cost	$ 7	51	$ 18	$ 100
Restricted stock | General and administrative
Stock option and incentive plans
Total compensation cost	$ 11	$ 128	$ 51	$ 273
2005 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	0		0
2005 Plan | Stock options
Stock Options
Outstanding (in shares)	362,112		362,112
2014 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	1,256,003		1,256,003
2014 Plan | Stock options
Stock Options
Outstanding (in shares)	4,318,997		4,318,997
2014 Plan | Restricted stock
Shares
Outstanding (in shares)	117,867		117,867