Commitments and Contingencies - Additional Information (Detail) £ in Thousands			1 Months Ended						3 Months Ended	12 Months Ended
		Jun. 07, 2016 USD ($)	Sep. 30, 2016 USD ($)	May 31, 2016 USD ($)	Apr. 30, 2016 USD ($)	Nov. 30, 2015 USD ($)	Jun. 30, 2015 GBP (£)	Apr. 30, 2015	Dec. 31, 2016 USD ($) Lease	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Lease Program	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Contingencies And Commitments [Line Items]
Number of non-cancellable operating leases | Lease												5
Number of expired leases | Lease									3			3
Lease expiration year												2017
Rental expense												$ 4,200,000	$ 1,300,000	$ 17,000
Deferred rent current									$ 1,027,000			1,027,000
Deferred rent non-current									12,283,000			12,283,000	164,000
Letters of credit secured by cash held in restricted depository account									3,150,000			3,150,000	700,000
Research and development expense	[1]											42,238,000	12,573,000	1,513,000
Cost sharing expensed relating to patent maintenance, defense and prosecution, incurred												3,000,000	1,500,000	$ 1,100,000
Cost sharing expensed relating to patent maintenance, defense and prosecution, accrued												2,800,000	2,600,000
Anagenesis Biotechnologies SAS [Member]
Contingencies And Commitments [Line Items]
Research and development expense												$ 500,000
Upfront payment		$ 500,000
Payment regarding achievement of future clinical, regulatory and sales milestones		$ 89,000,000
Sponsored Research Agreement [Member]
Contingencies And Commitments [Line Items]
Number of research programs | Program												5
Number of research programs continuing through 2018 | Program												2
Scenario Forecast [Member] | Sponsored Research Agreement [Member]
Contingencies And Commitments [Line Items]
Research and development expense										$ 100,000	$ 700,000
Formal Settlement Payment to US Common Shareholders [Member] | Maximum [Member]
Contingencies And Commitments [Line Items]
Aggregate settlement amount			$ 2,000,000
Formal Settlement Offer to US Common Shareholders [Member]
Contingencies And Commitments [Line Items]
Aggregate settlement amount									2,000,000
Letter of Credit [Member]
Contingencies And Commitments [Line Items]
Letters of credit secured by cash held in restricted depository account									3,200,000			$ 3,200,000	$ 700,000
Operating Lease, Research Facility Space [Member]
Contingencies And Commitments [Line Items]
Lease expiration date												Feb. 28, 2022
Extended term of lease expiration												5 years
Operating Lease, Primary Office and Research Facility Space [Member]
Contingencies And Commitments [Line Items]
Lease expiration date												Dec. 31, 2026
200 Sidney Street Lease [Member]
Contingencies And Commitments [Line Items]
Lease expiration date								Feb. 28, 2022
Extended term of lease expiration								5 years
Office Space Lease, London [Member]
Contingencies And Commitments [Line Items]
Lease expiration date							Jul. 31, 2017
Security cash deposit | £							£ 9
Corporate housing, Cambridge [Member]
Contingencies And Commitments [Line Items]
Lease expiration date						Nov. 30, 2017
Extended term of lease expiration						1 year
Security cash deposit						$ 10,000
610 Main Street Sublease [Member]
Contingencies And Commitments [Line Items]
Lease expiration date				Dec. 22, 2026	Jan. 31, 2017
Extended term of lease expiration				5 years
Rental expense												$ 2,300,000
Security cash deposit					$ 26,000
Tenant improvements allowance												10,800,000
Deferred rent liability									12,900,000			12,900,000
Tenant improvement allowance									10,200,000			10,200,000
Deferred rent current									1,000,000			1,000,000
Deferred rent non-current									$ 11,900,000			$ 11,900,000
610 Main Street Sublease [Member] | Letter of Credit [Member]
Contingencies And Commitments [Line Items]
Letters of credit secured by cash held in restricted depository account				$ 2,500,000

[1] Including the following amounts of research and development from a related party, see Note 16: