Assumptions Used in Estimating the Grant Date Fair Value of Options Using the Black-Scholes Option Pricing Model (Detail)	6 Months Ended
Jun. 30, 2012 years	Jun. 30, 2011 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	6	6
Dividend yield	0.91%	0.67%
Expected stock price volatility	39.88%	34.73%
Risk-free interest rate (weighted average)	1.07%	2.61%

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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