Commitments and Contingencies - Additional Information (Detail) (USD $) Share data in Thousands, unless otherwise specified	12 Months Ended									1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Other License Agreement [Member]	Dec. 31, 2012 Other License Agreement [Member]	Dec. 31, 2011 Other License Agreement [Member]	Dec. 31, 2013 University of Georgia Research Foundation [Member]	Dec. 31, 2012 University of Georgia Research Foundation [Member]	Dec. 31, 2011 University of Georgia Research Foundation [Member]	Apr. 30, 2013 Shire Human Genetic Therapies Inc. and Affiliates [Member] Milestone	Jun. 30, 2013 Shire Human Genetic Therapies Inc. and Affiliates [Member]	Dec. 31, 2013 Other Licensing Agreements [Member]
Operating Leases Of Lessee Future Minimum Payment Term Disclosure [Line Items]
Lease expiry period	2024
Rental expense	$ 1,600,000	$ 1,000,000	$ 700,000
Amendment agreement date							Apr. 05, 2007
Shares issued in consideration							9
Payments under license agreement							0	0	0
Acquired in-process research and development	2,500,000	394,000								2,500,000
Purchase of in-process research and development	2,500,000	344,000									2,500,000
Number of sales based milestones obligation										2
Agreements expenses description				Less than $0.1 million	Less than $0.1 million	Less than $0.1 million
Expenses incurred under license agreement				100,000	100,000	100,000
Potential milestone payments under license agreement				$ 100,000
Period of potential milestone payments under license agreement												4 years