Stock Options - Additional Information (Detail) (USD $)	9 Months Ended		94 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options available to purchase/Common Stock outstanding	7,403,854		7,403,854	8,648,268
Exercisable period	7 years 3 months 22 days
Stock-based compensation expense, net of estimated forfeitures	$ 2,201,039	$ 823,617	$ 5,234,350
Weighted average grant-date fair value of options granted	$ 2.71
Total fair value of options that vested	1,574,820
Total unrecognized compensation expense, related to non-vested share-based compensation arrangements	4,917,736		4,917,736
Period for unrecognized compensation expense is estimated to be recognized	3 years
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense, net of estimated forfeitures	$ 2,201,039	$ 823,617
Expected term of options granted	10 years
Weighted Average Exercise Price, Vested	48 months
2007 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options available to purchase/Common Stock outstanding	2,814,583		2,814,583
Shares available for future grants	0		0
Exercisable period	10 years
2010 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options available to purchase/Common Stock outstanding	4,589,271		4,589,271
Shares authorized for issuance	11,000,000		11,000,000
Shares available for future grants	6,342,918		6,342,918
Exercisable period	10 years