Schedule of Weighted Average Option, Performance Share Fair Values and Assumptions (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Weighted average fair value of option grants	$ 12.85	$ 11.47
Dividend yield	1.57%	1.57%
Volatility	30.94%	32.01%
Expected life (in years)	5 years 10 months 13 days	5 years 11 months 23 days
Performance Shares
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Weighted average fair value of option grants	$ 50.45
Dividend yield	0.00%
Volatility	28.99%
Risk-free forward interest rate	0.40%
Expected life (in years)	2 years 10 months 13 days
Minimum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	0.17%	0.17%
Maximum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	4.10%	3.98%