Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Each Significant Agreement (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	[1]	$ 390,635	$ 382,392	$ 276,495
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		184,703	160,580	115,768
MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		5,527	1,275	4,575
Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		3,250	525	3,300
Clinical trial and manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		174,901	150,179	108,903
Clinical trial and manufacturing | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		5,421	1,211	3,308
External services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		4,475	7,366	3,151
External services | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses				770
External services | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		3,250
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		5,327	3,035	3,714
Other | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses		$ 106	64	497
Other | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses			$ 525	$ 3,300

[1] Stock-based compensation expenses included in operating expenses are as follows: Research and development $51,872 $42,946 $27,086 General and administrative 40,947 32,582 18,697