Collaboration Agreement - Schedule of Collaboration Revenue Related to Upfront and Milestone Payments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amortization of deferred upfront and development milestones			$ 8,465	$ 16,931
Total collaboration revenue attributable to up-front and milestone payments	66,232	28,466	85,465	36,931
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to up-front and milestone payments	66,232	28,466	85,465	36,931
Astellas Pharma Inc. [Member] | Development milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development milestones earned	62,000	20,000	77,000	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	$ 4,232	$ 8,466	$ 8,465	$ 16,931