Stock Option and Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Stock option and incentive plans
Total stock-based compensation expense	$ 597	$ 216	$ 1,368	$ 263
Research and development
Stock option and incentive plans
Total stock-based compensation expense	179	35	436	39
General and administrative
Stock option and incentive plans
Total stock-based compensation expense	$ 418	$ 181	$ 932	$ 224
Stock option
Stock option and incentive plans
Stock options outstanding	1,757,326		1,757,326
Weighted average exercise price (in dollars per share)	$ 7.49		$ 7.49
Granted of stock options (in shares)			458,800
Weighted average grant date fair value (in dollars per share)			$ 14.18
Exercise of stock options (in shares)			327,098
Exercised (in dollars per share)			$ 1.14
Forfeited options (in shares)			44,950
Weighted average price of forfeited options (in dollars per share)			$ 6.13
2005 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	0		0
Stock options outstanding	634,387		634,387
2014 Plan
Stock option and incentive plans
Common stock reserved for issuance (in shares)	131,371		131,371
Stock options outstanding	1,122,939		1,122,939