Share-Based Payments - Recorded Stock-Based Compensation Expense for Employee and Non-Employee Stock Options and Restricted Stock (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$ 329,000	$ 91,000	$ 753,000	$ 189,000
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	246,000	60,000	533,000	126,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share based compensation expense	$ 83,000	$ 31,000	$ 220,000	$ 63,000