Summary of Activity Under Stock Option Plans (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012 Year
Options Outstanding
Beginning Balance	2,401,809
Options granted	46,950
Options exercised	(3,960)
Options cancelled	(3,800)
Ending Balance	2,440,999
Options exercisable at March 31, 2012	1,601,981
Options vested or expected to vest at March 31, 2012	2,413,888	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance	$ 37.54
Options granted	$ 41.27
Options exercised	$ 28.95
Options cancelled	$ 47.19
Ending Balance	$ 37.59
Options exercisable at March 31, 2012	$ 42.12
Options vested or expected to vest at March 31, 2012	$ 37.71	[1]
Weighted-Average Remaining Contractual Life in Years
Beginning Balance	5.0
Ending Balance	4.9
Options exercisable at March 31, 2012	3.4
Options vested or expected to vest at March 31, 2012	4.9	[1]
Aggregate Intrinsic Value
Beginning Balance	$ 12,195,931
Ending Balance	14,667,018
Options exercisable at March 31, 2012	5,404,008
Options vested or expected to vest at March 31, 2012	$ 14,389,127	[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.