Stock Option Information Related to Equity Incentive Plan (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012
Options
Outstanding, beginning of period	2,439,360
Granted	395,563
Exercised	(680,929)
Forfeited	(56,996)
Expired	(4,568)
Outstanding, end of period	2,092,430
Vested and expected to vest, end of period	2,003,907
Exercisable, end of period	1,088,716
Weighted average exercise price
Outstanding, beginning of period	$ 42.64
Granted	$ 63.90
Exercised	$ 35.28
Forfeited	$ 47.90
Expired	$ 42.50
Outstanding, end of period	$ 48.91
Vested and expected to vest, end of period	$ 48.50
Exercisable, end of period	$ 44.35
Weighted Average Remaining Contractual Life in Years
Outstanding, end of period	4 years 1 month 21 days
Vested and expected to vest, end of period	4 years 0 months 26 days
Exercisable, end of period	2 years 10 months 17 days
Aggregate Intrinsic Value of In-The-Money Options
Outstanding, end of period	$ 25,582,079
Vested and expected to vest, end of period	25,221,007
Exercisable, end of period	$ 17,542,729