Summary by Exercise Price of Outstanding Options, Options Vested and Options Expected to Vest (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Range, Lower Limit	$ 0.01
Exercise Price Range, Upper Limit	$ 10.00
Stock Options Outstanding	9,488,881	10,255,222
Weighted Average Remaining Contractual Life of Outstanding Stock Options (years)	8 years 3 months
Number of Stock Options Vested	3,125,677
Number of Stock Options Vested and Expected to Vest	9,089,708
$0.01 - $1.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Range, Lower Limit	$ 0.01
Exercise Price Range, Upper Limit	$ 1.00
Stock Options Outstanding	5,152,706
Weighted Average Remaining Contractual Life of Outstanding Stock Options (years)	7 years 4 months 24 days
Number of Stock Options Vested	2,724,812
Number of Stock Options Vested and Expected to Vest	5,049,203
$1.01 - $5.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Range, Lower Limit	$ 1.01
Exercise Price Range, Upper Limit	$ 5.00
Stock Options Outstanding	3,187,925
Weighted Average Remaining Contractual Life of Outstanding Stock Options (years)	9 years 1 month 17 days
Number of Stock Options Vested	388,654
Number of Stock Options Vested and Expected to Vest	2,989,216
$5.01 - $10.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Range, Lower Limit	$ 5.01
Exercise Price Range, Upper Limit	$ 10.00
Stock Options Outstanding	1,148,250
Weighted Average Remaining Contractual Life of Outstanding Stock Options (years)	9 years 7 months 13 days
Number of Stock Options Vested	12,211
Number of Stock Options Vested and Expected to Vest	1,051,289