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Stock based Compensation (Details) (USD $)	12 Months Ended
Stock based Compensation (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 7.89	$ 7.84	$ 6.11
Risk free interest rate	1.10%	1.40%	1.70%
Expected volatility	31.40%	34.10%	33.90%
Expected dividend yield	2.10%	2.00%	2.10%
Expected option life	6 years 7 months	5 years 10 months	3 years 7 months

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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