Commitments and Contingencies - Additional Information (Detail)		3 Months Ended		6 Months Ended
	Nov. 11, 2015 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued		$ 0		$ 0		$ 0
Office and Laboratory Space [Member]
Other Commitments [Line Items]
Rental expense related to office and laboratory space		1,145,000	$ 787,000	$ 2,244,000	$ 1,180,000
BMR Sidney Research Campus LLC [Member]
Other Commitments [Line Items]
Non-cancelable property lease, description				On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016
Square feet of property subject to non-cancelable lease | ft²	83,396
Lease commencement period				2016-03
Lease expiration period				2023-11
Cost of tenant improvements	$ 12,509,000
Lease incentive obligation		$ 11,196,000		$ 11,196,000
Amortized of lease incentive obligation				$ 884,000