Stockholders' Equity - Summary of the Company's Stock Options (Detail) (Employee Stock Option [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options, Options outstanding, beginning of period	6,321,076
Number of options, Granted
Number of options, Exercised	(1,985,596)
Number of options, Expired / forfeited	(132,288)
Number of options, Options outstanding, end of period	4,203,192
Number of options, Options exercisable, end of period	3,754,714
Number of options, Options expected to vest, end of period	380,339
Weighted average exercise price, Options outstanding, beginning of period	$ 6.05
Weighted average exercise price, Granted
Weighted average exercise price, Exercised	$ 5.14
Weighted average exercise price, Expired / forfeited	$ 5.72
Weighted average exercise price, Options outstanding, end of period	$ 6.49
Weighted average exercise price, Options exercisable, end of period	$ 6.71
Weighted average exercise price, Options expected to vest, end of period	$ 4.60
Weighted Average Remaining Contractual Life (years), Options outstanding, end of period	4 years 6 months 4 days
Weighted Average Remaining Contractual Life (years), Options exercisable, end of period	4 years 3 months 22 days
Weighted Average Remaining Contractual Life (years), Options expected to vest, end of period	6 years 2 months 9 days
Aggregate intrinsic value, Options outstanding, end of period	$ 35,702,302
Aggregate intrinsic value, Options exercisable, end of period	31,097,408
Aggregate intrinsic value, Options expected to vest, end of period	$ 3,908,919