Outstanding Options, Options Vested and Options Vested and Expected to Vest (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Share Based Compensation [Line Items]
Stock Options Issued and Outstanding, Options Outstanding	10,438,037	10,255,222
Stock Options Issued and Outstanding, Options Vested	2,949,875
Stock Options Issued and Outstanding, Options Vested and Expected to Vest	9,944,411
Weighted Average Remaining Contractual Life, Options Outstanding	8 years 5 months 27 days
Weighted Average Remaining Contractual Life, Options Vested	7 years 3 months 29 days
Weighted Average Remaining Contractual Life, Options Vested and Expected to Vest	8 years 5 months 16 days
Weighted Average Exercise Price, Options Outstanding	$ 1.98	$ 1.19
Weighted Average Exercise Price, Options Vested	$ 0.61
Weighted Average Exercise Price, Options Vested and Expected to Vest	$ 1.92