Collaboration Agreements - Substantive Development Milestone Payments Eligible to Receive under Astellas Collaboration Agreement (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Abstract]
Payment for triggering event	$ 7.0
Milestone payments received	$ 78.0