Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 129,127	$ 96,857
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	5,020	13,705
Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,672	641
Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	659	1,442
Clinical trial and manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	4,826	10,523
Clinical trial and manufacturing | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,612	641
Clinical trial and manufacturing | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	294	554
External services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	135	2,673
External services | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	10
External services | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	236	688
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	59	509
Other | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	50
Other | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 129	$ 200