Collaboration Agreements - Schedule of Collaboration Revenue Related to Upfront and Milestone Payments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestones earned	$ 50,000	$ 25,000
Amortization of deferred upfront and development milestones			14,109	25,396	100,944
Total collaboration revenue attributable to upfront and milestone payments			321,109	70,396	145,944
Astellas Pharma Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to upfront and milestone payments			321,109	70,396	73,914
Astellas Pharma Inc. [Member] | Development milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestones earned			257,000	20,000	45,000
Astellas Pharma Inc. [Member] | Sales milestones earned [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales milestones earned			50,000	25,000
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			14,109	25,396	28,914
Pfizer [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total collaboration revenue attributable to upfront and milestone payments			321,109	70,396	145,944
Pfizer [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					$ 72,030