Commitments and Contingencies - Additional Information (Detail)			12 Months Ended
	Nov. 11, 2015 USD ($) ft²	Apr. 02, 2015	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016 USD ($)
Other Commitments [Line Items]
Operating lease agreement expiration date			Jun. 30, 2016
Operating lease termination write off of deferred rent benefit			$ 136,000
Outstanding security deposit						$ 119,000
Lease incentive obligation			10,730,000	$ 586,000
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued			$ 0	0
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			$ 12,080,000
Amortized of lease incentive obligation			429,000
Office and Laboratory Space [Member]
Other Commitments [Line Items]
Operating lease agreement expiration date		Apr. 30, 2020
Rental expense related to office and laboratory space			$ 3,826,000	$ 1,246,000	$ 543,000