Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of Shares, Outstanding, Beginning balance	20,107
Number of Shares, Granted	3,704
Number of Shares, Exercised	(2,793)
Number of Shares, Forfeited	(471)
Number of Shares, Outstanding, Ending balance	20,547	20,107
Number of shares vested and expected to vest	20,220
Number of stock option exercisable	15,313
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 3.93
Weighted Average Exercise Price, Granted	$ 5.45
Weighted Average Exercise Price, Exercised	$ 2.27
Weighted Average Exercise Price, Forfeited	$ 6.30
Weighted Average Exercise Price, Outstanding, Ending balance	$ 4.37	$ 3.93
Weighted average exercise price of shares vested and expected to vest (in dollars per share)	$ 4.35
Weighted average exercise price for exercisable option (in dollars per share)	$ 3.84
Weighted Average Remaining Contractual Term	6 years 4 months 17 days	6 years 1 month 10 days
Weighted Average Remaining Contractual Term, Vested and expected to vest	6 years 3 months 29 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 6 months
Aggregate Intrinsic Value, Outstanding, Beginning balance	$ 38,348
Aggregate Intrinsic Value, Outstanding, Ending balance	90,628	38,348
Aggregate Intrinsic Value, Vested and expected to vest	89,628
Aggregate Intrinsic Value, Exercisable	$ 75,699