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Fair Value of Options Granted (Detail) (Stock Options, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock Options
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Risk-free interest rate	1.04%	2.29%	2.66%
Expected dividend yield	2.79%	2.59%	2.65%
Expected stock price volatility	33.35%	30.88%	30.40%
Expected life of options (in years)	6 years	6 years	6 years
Estimated fair value per share	$ 5.62	$ 5.56	$ 5.20

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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