Collaboration Agreements - Substantive Development Milestone Payments Eligible to Receive under Astellas Collaboration Agreement (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Abstract]
Payment for triggering event	$ 7
Development milestone payments earned to date	78.0
Development milestone payments received to date	$ 63.0