Collaboration Agreement - Substantive Development Milestone Payments Eligible to Receive under Astellas Collaboration Agreement (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	1 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2014	May 31, 2014 Subsequent Event [Member]	Apr. 30, 2014 Subsequent Event [Member]	Jun. 30, 2014 Scenario, Forecast [Member]	Mar. 31, 2014 Astellas Pharma Inc. [Member] Development milestone payments [Member]	Jun. 30, 2014 Astellas Pharma Inc. [Member] Scenario, Forecast [Member] Development milestone payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments earned	$ 78.0	$ 25.0	$ 15.0	$ 7.0		$ 47.0
Milestone payments received	78.0
Receivable from collaboration partner					$ 15.0