Commitments and contingencies - Additional Information (Detail) € in Millions				1 Months Ended	3 Months Ended			12 Months Ended
	Nov. 18, 2016 EUR (€)	Jun. 03, 2016 USD ($)	Sep. 21, 2015	Mar. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2016 EUR (€)	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 | €							€ 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.
Decrease in letter of credit under the terms of the lease						$ 9,200,000
Estimated future payments for 2018				$ 12,000,000		12,000,000
Estimated future payments for 2019				12,000,000		12,000,000
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
Letter of credit under the terms of lease						$ 13,800,000
Strategic 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				$ 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
Strategic manufacturing agreement [Member] | Maximum [Member]
Commitments And Contingencies Disclosure [Line Items]
Additional contractual payables to milestones		$ 8,000,000
Strategic manufacturing agreement [Member] | Scenario Forecast [Member]
Commitments And Contingencies Disclosure [Line Items]
Milestone payments					$ 2,000,000
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Commitments And Contingencies Disclosure [Line Items]
Lease expiration date			Mar. 31, 2027