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	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to up-front and milestone payments	$ 70,396	$ 145,944	$ 60,389
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to up-front and milestone payments	70,396	73,914	24,374
Pfizer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to up-front and milestone payments		$ 72,030	$ 36,015