Stock-based Compensation - Stock-based Compensation Expense for Options Granted and Restricted Stock as Reflected in the Statement of Operations (Detail) (USD $)	3 Months Ended		9 Months Ended		99 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,130,000	$ 108,000	$ 1,531,000	$ 292,000	$ 2,561,000
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	62,000	22,000	105,000	66,000	284,000
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,068,000	$ 86,000	$ 1,426,000	$ 226,000	$ 2,277,000