Collaboration Agreement - Substantive Development Milestone Payments Eligible to Receive under Astellas Collaboration Agreement (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended	3 Months Ended	1 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2014 Additional milestone payment [Member]	Oct. 31, 2014 Subsequent Event [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments earned	$ 90.0	$ 55.0	$ 15.0	$ 78.0	$ 7.0	$ 45.0