Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Liabilities (Detail) (Level 3 [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015
Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2014	$ 84,920
Change in fair value of success payments liabilities attributable to the elapsed service period recorded in research and development expense	38,910
Balance at March 31, 2015	$ 123,830

Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration