Schedule of Weighted Average Option Fair Values and Assumptions (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Weighted average fair value of option grants	$ 9.55	$ 11.66	$ 11.40	$ 12.31
Dividend yield	2.07%	1.63%	1.59%	1.45%
Volatility	31.96%	31.84%	32.01%	32.72%
Expected life (in years)	5 years 10 months 21 days	6 years 29 days	5 years 11 months 19 days	6 years 1 month 21 days
Minimum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	0.18%	0.18%	0.17%	0.28%
Maximum
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free forward interest rate	3.34%	5.15%	3.97%	5.62%