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Weighted-Average Assumptions used to Estimate Fair Value of Each Option Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	1.59%	1.53%	1.66%
Volatility	34.04%	33.04%	33.13%
Average expected life (years)	6 years	6 years	6 years
Risk-free interest rate	2.05%	3.67%	3.92%
Fair value of options granted	$ 20.00	$ 18.42	$ 13.76

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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