Stock Option and Incentive Plans - Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Stock option and incentive plans
Total stock-based compensation expense	$ 739	$ 383
Research and development
Stock option and incentive plans
Total stock-based compensation expense	233	127
General and administrative
Stock option and incentive plans
Total stock-based compensation expense	$ 506	$ 256
Stock option
Stock option and incentive plans
Stock options outstanding	1,731,722
Weighted average exercise price (in dollars per share)	$ 7.98
2005 Plan \| Stock option
Stock option and incentive plans
Stock options outstanding	430,922
Common stock reserved for issuance (in shares)	0
2014 Plan \| Stock option
Stock option and incentive plans
Stock options outstanding	1,300,800
Common stock reserved for issuance (in shares)	724,773

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 14
-Section F

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
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No definition available.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 718
-SubTopic 10
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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