Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 2,069	$ 17,096	$ 53,875	$ 51,988
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	1,609	17,096	53,415	51,988
Net Revenues from Collaborators | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	2,307		103,548
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	(1,560)	14,603	40,370	41,255
Net Revenues from Collaborators | Sanofi Genzyme | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	(862)		86,107
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	2,957		10,313
Net Revenues from Collaborators | Vir Biotechnology | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	2,957		10,313
Net Revenues from Collaborators | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues		2,255	1,957	10,141
Net Revenues from Collaborators | Mdco | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues			6,353
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	212	$ 238	775	$ 592
Net Revenues from Collaborators | Other | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 212		$ 775