Collaboration Agreement - Schedule of Collaboration Revenue Related to Upfront and Milestone Payments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amortization of deferred upfront and development milestones			$ 12,698	$ 21,163
Total collaboration revenue attributable to up-front and milestone payments	94,233	4,232	179,698	41,163
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to up-front and milestone payments	94,233	4,232	179,698	41,163
Astellas Pharma Inc. [Member] | Development milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestones earned	90,000		167,000	20,000
Astellas Pharma Inc. [Member] | Amortization of deferred upfront and development milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amortization of deferred upfront and development milestones	$ 4,233	$ 4,232	$ 12,698	$ 21,163