Stock Based Compensation - Summary of Activity Under Stock Option Plans (Detail) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012	Jun. 30, 2013		Jun. 30, 2012
Options Outstanding
Beginning Balance	1,670,647			1,765,947
Options granted	0			0		46,950
Options exercised	(156,057)			(251,240)
Options cancelled	(10,717)			(10,834)
Ending Balance	1,503,873		1,765,947	1,503,873
Options exercisable	1,201,313			1,201,313
Options vested or expected to vest	1,494,796	[1]		1,494,796	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance	$ 41.06			$ 40.58
Options granted	$ 0.00			$ 0.00		$ 41.27
Options exercised	$ 29.80			$ 30.71
Options cancelled	$ 36.17			$ 36.05
Ending Balance	$ 42.26		$ 40.58	$ 42.26
Options exercisable	$ 43.66			$ 43.66
Options vested or expected to vest	$ 42.29	[1]		$ 42.29	[1]
Weighted-Average Remaining Contractual Life in Years
Beginning Balance	3 years 8 months 12 days		4 years 3 months 18 days
Ending Balance	3 years 8 months 12 days		4 years 3 months 18 days
Options exercisable	2 years 10 months 24 days
Options vested or expected to vest	3 years 8 months 12 days	[1]
Aggregate Intrinsic Value
Beginning Balance	$ 15,558,730
Ending Balance	12,388,067			12,388,067
Options exercisable	9,078,139			9,078,139
Options vested or expected to vest	$ 12,288,769	[1]		$ 12,288,769	[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.