Commitments and contingencies - Additional Information (Detail) € in Millions					3 Months Ended		36 Months Ended
	Sep. 21, 2015 USD ($) ft² $ / ft²	Jun. 29, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Jun. 03, 2013 USD ($) ft²	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2014 EUR (€)	Mar. 31, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 EUR (€)
Commitments And Contingencies Disclosure [Line Items]
Other noncurrent assets						$ 10,686,000			$ 10,360,000
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
Option to extend this lease				5 years
Contribution from the landlord towards the initial build-out of the space				$ 7,800,000
Cash-collateralized irrevocable standby letter of credit				$ 1,300,000
Lease starting date				Jun. 03, 2013
Property and equipment, gross						104,796,000			89,435,000
Construction financing lease obligation						74,414,000			$ 61,901,000
Land [Member]
Commitments And Contingencies Disclosure [Line Items]
Non-cash rent expense						500,000
Construction In Progress 60 Binney Street [Member]
Commitments And Contingencies Disclosure [Line Items]
Property and equipment, gross						74,700,000
Cash paid to the landlord						$ 0
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 is 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 has 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 is subject to a 3% annual rent increase plus certain operating expenses and taxes.
Operating lease, rent increase percentage		3.00%
Lease starting on October 1, 2016 [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period	120 months
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 Lease, starting on October 1, 2016, the Company will lease 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 based on reimbursement of tenant improvement cost	$ 13,800,000
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Annual lease rent per square foot | $ / ft²	72.50
Lease rent increase percentage	1.75%
Option to extend capital lease	10 years
Scenario Forecast [Member]
Commitments And Contingencies Disclosure [Line Items]
Cash-collateralized irrevocable standby letter of credit					$ 600,000
French Credit d'Impot Recherche Program [Member]
Commitments And Contingencies Disclosure [Line Items]
Eligible percentage of research expense monetized						30.00%
Aggregate reimbursement received related to years 2012 through 2014 | €							€ 1.6
Other long term receivable | €										€ 1.0
Other noncurrent assets | €								€ 0.4
Other short term receivable | €								€ 1.0