Commitments and Contingencies - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 ft²
Operating Leased Assets [Line Items]
Area of additional office space leased | ft²					5,100
Deferred rent and lease incentive liability	$ 200,000			$ 200,000
Current portion of deferred rent and lease incentive liability (less than)	83,000			72,000
Rent expense	500,000	$ 500,000
Restricted cash and other assets | Letter of credit
Operating Leased Assets [Line Items]
Letter of credit	300,000.0
Accrued expenses
Operating Leased Assets [Line Items]
Current portion of deferred rent and lease incentive liability (less than)	$ 100,000			$ 100,000
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Tenant improvement allowance			$ 700,000