Stock Based Compensation (Details 1) (USD $)	12 Months Ended
Dec. 31, 2011 Y	Dec. 31, 2010 Y	Dec. 31, 2009 Y
Weighted-average assumptions used in determining the fair values of stock option
Stock price volatility	24.25%	25.73%	24.39%
Expected term in years	6.64	5.46	5.33
Risk-free interest rate	2.70%	2.57%	2.22%
Dividend yield	4.20%	3.74%	3.50%
Dividend payment per share	$ 3.32	$ 3.24	$ 3.00

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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