Collaboration Agreements - Substantive Development Milestone Payments Eligible to Receive under Astellas Collaboration Agreement (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Abstract]
Payment for triggering event	$ 7.0
Deferred revenue additions related to development milestones	$ 58.0