Commitments and contingencies - Additional Information (Detail) € in Millions				1 Months Ended	3 Months Ended					9 Months Ended		12 Months Ended
	Nov. 18, 2016 EUR (€)	Jun. 03, 2016 USD ($)	Sep. 21, 2015	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 EUR (€)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Term of agreement	5 years
Renewal period of agreement	3 years
Up-front fee | €	€ 3.0
Up-front fee paid | €						€ 1.0			€ 2.0
Annual fees					$ 29,000			$ 1,100,000		$ 67,000	$ 1,520,000
Termination description										The Company may terminate this agreement with twelve months’ notice and a one-time termination fee.
Decrease in letter of credit under the terms of the lease										$ 9,200,000
Estimated future payments for 2018					4,000,000					4,000,000
Estimated future payments for 2019					23,000,000					23,000,000
Pregenen [Member]
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments					120,000,000					120,000,000
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
Building [Member]
Commitments And Contingencies Disclosure [Line Items]
Useful life of asset										40 years
Interest expense related to financing obligation					$ 3,900,000			$ 4,100,000		$ 11,600,000	$ 7,800,000
Letter of credit under the terms of lease										$ 13,800,000
Maintenance and Production [Member]
Commitments And Contingencies Disclosure [Line Items]
Annual fees | €	€ 9.8
Manufacturing Agreement [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease period		12 years
Amount payable upon achievement of certain contractual milestones		$ 12,500,000
Milestone payments				$ 1,500,000			$ 2,000,000					$ 5,500,000	$ 5,000,000
Lease payments		$ 5,100,000
Agreement termination description										The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.
Early termination lease term		24 months
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] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Mar. 31, 2027