Commitments and contingencies - Additional Information (Detail) € in Millions									1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($)	Nov. 18, 2016 EUR (€)	Jun. 03, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Jun. 29, 2015 USD ($) ft²	Dec. 31, 2014 USD ($)	Jun. 03, 2013 USD ($) ft²	Jan. 31, 2017 USD ($)	Dec. 31, 2016 EUR (€)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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			$ 600,000				$ 1,300,000
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		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	169,399,000			89,435,000								$ 169,399,000	$ 89,435,000
Construction financing lease obligation	120,140,000			$ 61,901,000								120,140,000	61,901,000
Tenant improvement costs incurred, not reimbursable												1,800,000
Non-cash rent expense												8,300,000	5,700,000	$ 4,300,000
Pregenen [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	129,000,000											129,000,000
Settlement of contingent consideration liability												5,000,000	$ 1,000,000
Pregenen [Member] | Preclinical Milestones [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	9,000,000											9,000,000
Pregenen [Member] | Clinical Milestone Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	20,100,000											20,100,000
Pregenen [Member] | Commercial Milestones Payments [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments	99,900,000											99,900,000
Prepaid expenses and other current assets [Member]
Commitments And Contingencies Disclosure [Line Items]
Tenant improvement costs reimbursable not yet received	8,500,000											8,500,000
Construction In Progress 60 Binney Street [Member]
Commitments And Contingencies Disclosure [Line Items]
Property and equipment, gross	126,900,000											126,900,000
Construction financing lease obligation	$ 120,100,000											120,100,000
Cash paid to the landlord												0
Land [Member]
Commitments And Contingencies Disclosure [Line Items]
Non-cash rent expense												$ 1,900,000
Subsequent Event [Member]
Commitments And Contingencies Disclosure [Line Items]
Tenant improvement costs received									$ 7,800,000
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 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. The Company has provided its landlord with notice of termination of this lease, with termination to be effective April 12, 2017.
Operating lease, rent increase percentage						3.00%
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
Estimated year of construction completion												2018
Milestone payments made												$ 5,000,000
Agreement termination description												The 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] | Subsequent Event [Member]
Commitments And Contingencies Disclosure [Line Items]
Milestone payments made									$ 3,000,000
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 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 Street 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 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]
Annual lease rent per square foot | $ / ft²					72.50
Lease rent increase percentage					1.75%
Option to extend capital lease					5 years