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
	Mar. 31, 2013	Mar. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	$ 8,465	$ 36,825
Astellas [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total	8,465	5,937
Pfizer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total		$ 30,888