Stock Based Compensation - Summary of Activity Under Stock Option Plans (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Options Outstanding
Beginning Balance	1,765,947
Options exercised	(95,183)
Options cancelled	(117)
Ending Balance	1,670,647		1,765,947
Options exercisable at March 31, 2013	1,326,398
Options vested or expected to vest at March 31, 2013	1,660,320	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance	$ 40.58
Options exercised	$ 32.19
Options cancelled	$ 24.20
Ending Balance	$ 41.06		$ 40.58
Options exercisable at March 31, 2013	$ 42.40
Options vested or expected to vest at March 31, 2013	$ 41.09	[1]
Weighted-Average Remaining Contractual Life in Years
Beginning Balance	4 years 3 months		5 years
Ending Balance	4 years 3 months		5 years
Options exercisable at March 31, 2013	3 years 6 months
Options vested or expected to vest at March 31, 2013	4 years 2 months	[1]
Aggregate Intrinsic Value
Beginning Balance	$ 12,195,931
Ending Balance	15,558,730		12,195,931
Options exercisable at March 31, 2013	11,452,910
Options vested or expected to vest at March 31, 2013	$ 15,435,555	[1]

[1] * In addition to the vested options, we expect a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.