Collaboration Agreement - Schedule of Collaboration Revenue Related to Upfront and Milestone Payments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amortization of deferred upfront and development milestones			$ 2,257	$ 8,465
Total collaboration revenue attributable to upfront and milestone payments	$ 846	$ 66,232	2,257	85,465
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to upfront and milestone payments	846	66,232	2,257	85,465
Astellas Pharma Inc. [Member] | Development milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestones earned		62,000		77,000
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	$ 846	$ 4,232	$ 2,257	$ 8,465