Leases - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended				9 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Jan. 31, 2016	Oct. 31, 2014	Sep. 30, 2019	Dec. 31, 2018
Lessee Lease Description [Line Items]
Operating lease right-of-use assets					$ 20,663
Operating lease, liability					$ 19,647
130 Brookline Street [Member]
Lessee Lease Description [Line Items]
Operating lease, description					In October 2014, the Company entered into an agreement to lease office and laboratory space at 130 Brookline Street in Cambridge, Massachusetts under an operating lease agreement with a term through January 2020, with an option to extend the term of the lease for an additional five-year period. In April 2019, the Company executed an amendment to the lease to extend the term of the lease for the additional five-year period, through January 2025.
Operating lease expiration				2020-01
Lessee operating lease extended expiration date	Jan. 31, 2025
Operating lease, existence of option to extend				true
Operating lease, options to extend					option to extend the term of the lease for an additional five-year period. In April 2019, the Company executed an amendment to the lease to extend the term of the lease for the additional five-year period, through January 2025.
Operating lease, renewal term	5 years			5 years
Lessee operating sublease option to extend					two options to extend the agreement by one year each, for a total option period of up to two years.
Operating sublease expiration		2021-04
Operating sublease, existence of option to extend		true
Operating lease right-of-use assets	$ 1,300
Operating lease, liability	$ 1,300
130 Brookline Street [Member] | Maximum [Member]
Lessee Lease Description [Line Items]
Operating sublease, renewal term		2 years
130 Brookline Street [Member] | Other Assets [Member]
Lessee Lease Description [Line Items]
Lease security deposit				$ 300
40 Erie Street [Member]
Lessee Lease Description [Line Items]
Operating lease, description					In January 2016, the Company entered into a ten-year agreement to lease office and laboratory space at 40 Erie Street in Cambridge, Massachusetts under an operating lease agreement, with an option to terminate the lease at the end of the sixth year and an option to extend the term of the lease for an additional three years.
Operating lease, existence of option to extend			true
Operating lease, options to extend					option to extend the term of the lease for an additional three years.
Operating lease, renewal term			3 years
Operating lease, term of contract			10 years
Operating lease, existence of option to terminate			true
Operating lease, option to terminate					option to terminate the lease at the end of the sixth year
Operating lease, terminate term			6 years
Prepaid lease						$ 2,200
40 Erie Street [Member] | Other Assets [Member]
Lessee Lease Description [Line Items]
Lease security deposit			$ 2,200