Commitments and Contingencies - Additional Information (Detail)				1 Months Ended			6 Months Ended
	Apr. 07, 2017 ft²	Sep. 21, 2016 ft²	Jul. 13, 2015 ft²	Nov. 30, 2016 USD ($)	Oct. 31, 2016 USD ($)	May 31, 2013 USD ($)	Jun. 30, 2017	Jan. 01, 2018 ft²	Dec. 31, 2015 USD ($)
Commitment And Contingencies [Line Items]
Operating lease term						42 months
Minimum monthly lease payments | $						$ 17,588
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Additional leased office space	1,471	6,748	4,700
Office space leased on temporary agreement			6,700
Lease commencement date of extended office space			May 01, 2016
Lease termination date	Oct. 31, 2023	Oct. 31, 2017	Oct. 31, 2019
Extended leased office space			5,400
Extended lease agreement period			3 years
Lease termination description							The Company may terminate the amendment for convenience with nine months’ notice upon the occurrence of certain events connected to its clinical stage programs.
Operating lease early termination notice period			9 months
Lease commencement date		Oct. 01, 2016
Lease payment for additional space in incremental rent per month | $				$ 19,000	$ 18,300
Burlington Massachusetts [Member] | Scenario Forecast [Member]
Commitment And Contingencies [Line Items]
Additional leased office space								6,450
Letter of Credit [Member]
Commitment And Contingencies [Line Items]
Security deposit paid to lessor | $						$ 98,000			$ 50,000