Significant Agreements - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended		0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Oct. 12, 2007	May 28, 2009
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial upfront payment		$ 1.1
Additional payments to UWA on successful achievement		6
Collaboration and license agreement date	May 2003
Agreement expiration year	2006
Obligated to make milestone payments	26.8
Isis Pharmaceuticals, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Obligated to make milestone payments	21.1
Licensed patent expiration date	Mar. 27, 2028
Sarepta Therapeutics, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Licensed patent expiration date	Sep. 09, 2014
March 2008 Agreement [Member] | Ercole Biotechnology, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of share issued to acquire company, shares	5,811,721
Number of share issued to acquire company, value	8.4
Business acquisition liabilities assumed	1.8
Warrants issued to purchase common stock	0.4
Charley's Fund Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development grant awarded			2.5	5.0
Aggregate amount of research grant	3.4
Revenue recognized from research grant	0.1
Deferred grant payment	$ 3.3