Significant Accounting Policies - Summary of Revenues (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investment Holdings [Line Items]
Royalty revenue	$ 2,562	$ 3,317
Collaborative and other research and development revenues:
Collaborative and other research and development	14,769	22,976	19,643
Total revenues	17,331	26,293	19,643
U.S. Department of Health and Human Services [Member]
Collaborative and other research and development revenues:
Collaborative and other research and development	13,585	14,026	17,099
Shionogi & Co. Ltd [Member]
Collaborative and other research and development revenues:
Collaborative and other research and development	1,184	1,184	1,181
Mundipharma (United Kingdom) [Member]
Collaborative and other research and development revenues:
Collaborative and other research and development		7,766	1,277
Grants (United States) [Member]
Collaborative and other research and development revenues:
Collaborative and other research and development			$ 86