Collaboration Agreements - Schedule of Collaboration Revenue Attributable to Up-Front and Milestone Payments (Detail) (Up-front cash payment arrangement [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	$ 28,466	$ 42,912	$ 36,931	$ 79,737
Astellas [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	28,466	7,256	36,931	13,193
Pfizer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total		$ 35,656		$ 66,544