Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators	$ 37,924	$ 17,096	$ 15,932	$ 18,960	$ 17,454	$ 13,651	$ 8,709	$ 7,345	$ 89,912	$ 47,159	$ 41,097
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									54,625	32,015	11,005
MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									30,217	11,220	10,301
Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators											8,867
Monsanto
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									2,500		5,621
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									$ 2,570	$ 3,924	$ 5,303