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Summary of Significant Accounting Policies Summary of Significant Accounting Policies Compensation (Details) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate 27.40% 27.10% 32.40%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term 5 years 0 months 0 days 5 years 0 months 0 days 5 years 0 months 0 days
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate 4.35% 4.56% 4.85%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate 0.71% 2.27% 2.29%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 8.54 $ 8.36 $ 6.18
Historical Volatility Period Basis 10    
Stock Option [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 8.55 $ 8.18