Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Apr. 30, 2015 ft²
Sublease income	$ 0	$ 200,000	$ 700,000
Deferred rent and lease incentive liability	300,000	500,000
Current portion of deferred rent and lease incentive liability (less than in 2016)	29,000	405,000
Rent expense, net of sublease payments	1,600,000	1,100,000	$ 600,000
Restricted cash and other deposits | Letter of Credit
Letter of credit	300,000.0
Accrued expenses
Current portion of deferred rent and lease incentive liability (less than in 2016)	$ 100,000	$ 400,000
Laboratory and office space
Area exchanged (in square feet) | ft²					13,711
Laboratory and office space agreement amendment
Area exchanged (in square feet) | ft²					15,174
Maximum | Laboratory and office space
Tenant improvement allowance				$ 700,000.0