Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators	$ 17,454	$ 13,651	$ 8,709	$ 7,345	$ 7,551	$ 6,324	$ 8,685	$ 18,537	$ 47,159	$ 41,097	$ 50,561
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									32,015	11,005	369
MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									11,220	10,301	10,753
Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators										8,867	21,973
Monsanto
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators										5,621	14,985
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from collaborators									$ 3,924	$ 5,303	$ 2,481