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Employee Benefit Plans Assumptions Used in Estimating the Grant Date Fair Value of Options Using the Black-Scholes Option Pricing Model (Detail)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Dividend yield	1.28%	0.92%
Expected stock price volatility	30.98%	40.41%
Risk-free interest rate (weighted average)	1.02%	1.07%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	6 years	6 years

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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