Stock Based Compensation - Summary of Activity Under Stock Option Plans (Detail) (USD $)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Jun. 30, 2012	Sep. 30, 2012		Sep. 30, 2011
Options Outstanding
Beginning Balance	2,299,469			2,401,809	2,401,809
Options granted	0		50,000		46,950		108,550
Options exercised	(168,310)				(284,801)
Options cancelled	(32,950)				(65,749)
Ending Balance	2,098,209			2,299,469	2,098,209
Options exercisable at September 30, 2012	1,517,217				1,517,217
Options vested or expected to vest at September 30, 2012	2,078,839	[1]			2,078,839	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance	$ 38.07			$ 37.54	$ 37.54
Options granted	$ 0.00		$ 48.70		$ 41.27		$ 48.26
Options exercised	$ 40.34				$ 39.99
Options cancelled	$ 54.45				$ 43.31
Ending Balance	$ 38.74			$ 38.07	$ 38.74
Options exercisable at September 30, 2012	$ 41.52				$ 41.52
Options vested or expected to vest at September 30, 2012	$ 38.84	[1]			$ 38.84	[1]
Weighted-Average Remaining Contractual Life in Years
Options outstanding	4 years 7 months 7 days			4 years 9 months 20 days
Options exercisable at September 30, 2012	3 years 7 months 7 days
Options vested or expected to vest at September 30, 2012	4 years 7 months 7 days	[1]
Aggregate Intrinsic Value
Beginning Balance	$ 14,513,131
Ending Balance	15,874,392			14,513,131	15,874,392
Options exercisable at September 30, 2012	9,055,841				9,055,841
Options vested or expected to vest at September 30, 2012	$ 15,669,835	[1]			$ 15,669,835	[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.