Stock Option and Incentive Plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
2005 Stock option and incentive plans
Total stock based compensation expense	$ 383	$ 23
Research and development
2005 Stock option and incentive plans
Total stock based compensation expense	127	2
General and administrative
2005 Stock option and incentive plans
Total stock based compensation expense	$ 256	$ 21
Stock option
2005 Stock option and incentive plans
Stock options outstanding	1,478,662
Weighted average exercise price (in dollars per share)	$ 4.80
Granted of stock options (in shares)	0
Exercise of stock options (in shares)	191,912
Exercised (in dollars per share)	$ 1.04
2005 Plan
2005 Stock option and incentive plans
Common stock reserved for issuance (in shares)	0
Stock options outstanding	778,662
2014 Plan
2005 Stock option and incentive plans
Common stock reserved for issuance (in shares)	560,310
Stock options outstanding	700,000

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Aggregate number of common shares reserved for future issuance.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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