Share Based Payments - Summary of Stock Option Activity under all Equity Plans (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning balance, Number of Shares	2,529	2,371	1,244
Options granted, Number of Shares	1,119	820	1,029
Options assumed through merger with Trimeris, Number of Shares			373
Options exercised, Number of Shares	(660)	(519)	(209)
Options canceled or expired, Number of Shares	(278)	(143)	(66)
Ending balance, Number of Shares	2,710	2,529	2,371
Beginning balance, Weighted Average Exercise Price	$ 25.06	$ 15.34	$ 1.27
Exercisable, Number of Shares	1,000
Options granted, Weighted Average Exercise Price	$ 45.31	$ 43.90	$ 13.39
Exercisable and expected to vest, Number of Shares	2,521
Options assumed through merger with Trimeris, Weighted Average Exercise Price			$ 57.23
Options exercised, Weighted Average Exercise Price	$ 7.52	$ 2.62	$ 0.89
Options canceled or expired, Weighted Average Exercise Price	$ 60.80	$ 53.83	$ 1.41
Ending balance, Weighted Average Exercise Price	$ 34.05	$ 25.06	$ 15.34
Exercisable, Weighted Average Exercise Price	$ 26.39
Exercisable and expected to vest, Weighted Average Exercise Price	$ 33.71