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Shareholders' Equity (Schedule of Assumptions Used to Calculate Fair Value of Options Granted) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 Yr	Mar. 31, 2012 Yr
Shareholders' Equity
Risk-free interest rate	1.30%	1.50%
Dividend yield	3.60%	4.40%
Volatility factor	28.00%	33.00%
Expected life	7.0	7.0
Fair value of options per share	$ 5.48	$ 6.07

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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