Summary of Activity Under Stock Option Plans (Detail) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011
Options Outstanding
Beginning Balance	2,440,999		2,401,809		2,401,809
Options granted				58,550	46,950		58,550
Options exercised	(112,531)				(116,491)
Options cancelled	(28,999)				(32,799)
Ending Balance	2,299,469		2,440,999		2,299,469
Options exercisable at June 30, 2012	1,594,269				1,594,269
Options vested or expected to vest at June 30, 2012	2,273,222	[1]			2,273,222	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance	$ 37.59		$ 37.54		$ 37.54
Options granted				$ 47.89	$ 41.27		$ 47.89
Options exercised	$ 29.74				$ 29.72
Options cancelled	$ 46.78				$ 44.06
Ending Balance	$ 38.07		$ 37.59		$ 38.07
Options exercisable at June 30, 2012	$ 41.39				$ 41.39
Options vested or expected to vest at June 30, 2012	$ 38.19	[1]			$ 38.19	[1]
Weighted-Average Remaining Contractual Life in Years
Options outstanding	4 years 9 months 18 days		4 years 10 months 24 days
Options exercisable at June 30, 2012	3 years 8 months 12 days
Options vested or expected to vest at June 30, 2012	4 years 8 months 12 days	[1]
Aggregate Intrinsic Value
Beginning Balance	$ 14,667,018
Ending Balance	14,513,131		14,667,018		14,513,131
Options exercisable at June 30, 2012	7,279,532				7,279,532
Options vested or expected to vest at June 30, 2012	$ 14,247,752	[1]			$ 14,247,752	[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.