Stock Based Compensation - Stock Option Activity Under Equity Incentive Plan (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Outstanding Beginning Balance	6,526,506
Number of Shares, Granted	1,835,250
Number of Shares, Exercised	(67,303)
Number of Shares, Forfeitures	(121,593)
Number of Shares, Outstanding Ending Balance	8,172,860
Number of Shares, Vested and expected to vest	8,096,147
Number of Shares, Exercisable	4,806,759
Weighted-Average Exercise Price, Outstanding at beginning of period	$ 3.66
Weighted-Average Exercise Price, Granted	$ 3.81
Weighted-Average Exercise Price, Exercised	$ 1.43
Weighted-Average Exercise Price, Forfeitures	$ 4.67
Weighted-Average Exercise Price, Outstanding at end of period	$ 3.70
Weighted-Average Exercise Price, Vested and expected to vest	$ 3.69
Weighted-Average Exercise Price, Exercisable	$ 3.14
Weighted-Average Remaining Contractual Term, Outstanding	7 years 5 months 5 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	7 years 4 months 28 days
Weighted-Average Remaining Contractual Term, Exercisable	6 years 6 months 11 days
Aggregate Intrinsic Value, Outstanding	$ 6,734,462
Aggregate Intrinsic Value, Vested and expected to vest	6,731,205
Aggregate Intrinsic Value, Exercisable	$ 6,278,785