Commitments and contingencies - Additional Information (Detail) € in Millions							3 Months Ended					9 Months Ended
	Nov. 18, 2016 EUR (€)	Jun. 03, 2016 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Jun. 29, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Jun. 03, 2013 USD ($) ft²	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 EUR (€)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Lease period						9 years
Lease building space | ft²						43,600
Amendment effective date						2014-06
Additional lease building space | ft²						9,900
Lease payments						$ 200,000
Increase in monthly lease payments					$ 300,000
Contribution from the landlord towards the initial build-out of the space						$ 7,800,000
Cash-collateralized irrevocable standby letter of credit							$ 600,000
Lease starting date						Jun. 03, 2013
Lease termination date												Apr. 12, 2017
Term of agreement	5 years
Renewal period of agreement	3 years
Up-front fee | €	€ 3.0
Up-front fee paid | €										€ 2.0
Up-front fee payable in mid-2018 | €	1.0
Annual maintenance and production fees | €	€ 9.8
Termination description												The Company may terminate this agreement with six months’ notice and a one-time termination fee prior to July 1, 2018, or twelve months’ notice and a one-time termination fee thereafter.
Property and equipment, gross							195,597,000					$ 195,597,000		$ 169,399,000
Tenant improvements receivable							2,712,000					2,712,000		8,542,000
60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Property and equipment, gross							161,800,000					161,800,000		126,900,000
Construction financing lease obligation							154,800,000					154,800,000		120,100,000
Tenant improvements receivable							2,700,000					$ 2,700,000		$ 8,500,000
Building [Member]
Commitments And Contingencies Disclosure [Line Items]
Useful life of asset												40 years
Interest expense related to financing obligation							4,100,000					$ 7,800,000
Property and equipment, gross							161,817,000					161,817,000
Land [Member]
Commitments And Contingencies Disclosure [Line Items]
Non-cash rent expense							$ 500,000				$ 500,000	$ 1,400,000	$ 1,400,000
150 Second Street [Member]
Commitments And Contingencies Disclosure [Line Items]
Tenant improvement assets disposed								$ 8,300,000
Non-current deferred rent disposed								8,000,000
Loss of tenant improvement assets disposed								$ 300,000
Lease starting on July 13, 2015 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period				60 months
Lease building space | ft²				15,120
Lease payments				$ 500,000
Lease starting date				Jul. 13, 2015
Early termination lease term				20 months
Operating lease description												Under the terms of the lease, the Company leased approximately 15,120 square feet starting on July 13, 2015 for $0.5 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes.
Operating lease, rent increase percentage				3.00%
Lease starting on January 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease building space | ft²				8,075
Lease payments				$ 300,000
Lease starting date				Jan. 01, 2016
Operating lease description												Under the terms of the lease, the Company also leased an additional 8,075 square feet of office space in the same premises starting on January 1, 2016 for an additional $0.3 million per year in base rent, which was subject to a 3% annual rent increase plus certain operating expenses and taxes. The Company terminated this lease effective April 12, 2017.
Operating lease, rent increase percentage				3.00%
Lease expiration date				Aug. 31, 2020
Strategic manufacturing agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period		12 years
Lease payments		$ 5,100,000
Early termination lease term		24 months
Amount payable upon achievement of certain contractual milestones		$ 12,500,000
Milestone payments made		5,000,000							$ 3,000,000
Agreement termination description												Company may terminate this agreement any time after July 1, 2016 upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.
Strategic manufacturing agreement [Member] | Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Additional contractual payables to milestones		$ 8,000,000
Lease starting on October 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease building space | ft²			253,108
Contribution from the landlord towards the initial build-out of the space			$ 42,400,000
Cash-collateralized irrevocable standby letter of credit			$ 9,200,000
Lease starting date			Oct. 01, 2016
Lease payments base annual rent			$ 18,400,000
Capital lease description												Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes.
Increase in letter of credit under the terms of the lease											$ 13,800,000
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Mar. 31, 2027
Annual lease rent per square foot | $ / ft²			72.50
Lease rent increase percentage			1.75%
Option to extend capital lease			10 years