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Stock-Based Compensation (Details 4)	3 Months Ended
Stock-Based Compensation (Details 4)	Mar. 31, 2013	Mar. 31, 2012
Assumptions used for computing share-based employee compensation expense
Risk-free interest rate	1.95%	1.95%
Dividend yield	2.00%	2.00%
Stock price volatility	20.21%	38.75%
Expected term	10 years	10 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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