Revenue from Research Collaborators (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators	$ 8,495	$ 16,759	$ 20,884	$ 20,587	$ 20,455	$ 20,791	$ 20,614	$ 20,897	$ 66,725	$ 82,757	$ 100,041
Roche/Arrowhead
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									37,318	55,978	55,978
Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									21,973	22,248	22,250
Cubist
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									2,777	2,467	2,363
Monsanto
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									1,954
Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									60	149	9,313
Government contract
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators										152	4,335
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total net revenues from research collaborators									$ 2,643	$ 1,763	$ 5,802