Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Each Significant Agreement (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	[1]	$ 96,857	$ 86,984
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		13,705	42,578
Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		641	5,119
Clinical trial and manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		10,523	40,575
Clinical trial and manufacturing | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		641	5,095
External services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		2,673	567
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		$ 509	1,436
Other | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses			$ 24

[1] Stock-based compensation expenses included in operating expenses are as follows: Research and development $10,137 $8,691 General and administrative 9,447 7,026