Collaboration Agreement - Schedule of Collaboration Revenue Related to Upfront and Milestone Payments (Detail) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestones earned	$ 70,000	$ 50,000		$ 25,000
Amortization of deferred upfront and development milestones					$ 2,822	$ 12,698
Total collaboration revenue attributable to upfront and milestone payments	70,565		$ 94,233		72,822	179,698
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to upfront and milestone payments	70,565		94,233		72,822	179,698
Astellas Pharma Inc. [Member] | Sales milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestones earned	70,000				70,000
Astellas Pharma Inc. [Member] | Development milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestones earned			90,000			167,000
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	$ 565		$ 4,233		$ 2,822	$ 12,698