Share-Based Compensation - Stock Option Information Related to Equity Incentive Plan (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013
Options
Outstanding, beginning of period	2,060,413
Granted	0
Exercised	(265,814)
Forfeited	(8,769)
Expired	(200)
Outstanding, end of period	1,785,630
Vested and expected to vest, end of period	1,725,072
Exercisable, end of period	831,168
Weighted Average Exercise Price
Outstanding, beginning of period	$ 49.15
Granted	$ 0.00
Exercised	$ 48.53
Forfeited	$ 52.95
Expired	$ 46.48
Outstanding, end of period	$ 49.22
Vested and expected to vest, end of period	$ 48.84
Exercisable, end of period	$ 43.23
Weighted Average Remaining Contractual Life in Years
Outstanding, end of period	4 years 18 days
Vested and expected to vest, end of period	4 years
Exercisable, end of period	2 years 10 months 21 days
Aggregate Intrinsic Value of In-The- Money Options
Outstanding, end of period	$ 38,785,843
Vested and expected to vest, end of period	38,116,691
Exercisable, end of period	$ 23,032,874