Schedule of Weighted Average Option Fair Values and Assumptions (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Weighted average fair value of option grants	$ 11.23	$ 12.89	$ 11.44	$ 12.31
Dividend yield	1.83%	1.35%	1.58%	1.45%
Volatility	31.91%	31.96%	32.01%	32.73%
Expected life (in years)	5 years 11 months 1 day	5 years 10 months 21 days	5 years 11 months 23 days	6 years 1 month 17 days
Minimum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	0.19%	0.26%	0.17%	0.28%
Maximum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	3.99%	5.62%	3.98%	5.62%