Outstanding Options, Options Vested and Options Vested and Expected to Vest (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Options Issued and Outstanding, Options Outstanding	9,488,881	10,255,222
Stock Options Issued and Outstanding, Options Vested	3,125,677
Stock Options Issued and Outstanding, Options Vested and Expected to Vest	9,089,708
Weighted Average Remaining Contractual Life, Options Outstanding	8 years 3 months
Weighted Average Remaining Contractual Life, Options Vested	7 years 2 months 16 days
Weighted Average Remaining Contractual Life, Options Vested and Expected to Vest	8 years 2 months 19 days
Weighted Average Exercise Price, Options Outstanding	$ 2.03	$ 1.19
Weighted Average Exercise Price, Options Vested	$ 0.76
Weighted Average Exercise Price, Options Vested and Expected to Vest	$ 1.98