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Schedule of Fair Value of Options Granted (Detail)	9 Months Ended
Schedule of Fair Value of Options Granted (Detail)	Sep. 30, 2012	Sep. 30, 2011
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Risk-free interest rate	1.20%	2.70%
Dividend yield	2.70%	1.80%
Expected volatility of the Company's common stock	0.644%	0.615%
Expected life in years	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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