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Retirement and Share-based Compensation Plans (Stock Option Plans) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options outstanding, intrinsic value $ 21.3    
Options vested and expected to vest, intrinsic value 21.3    
Proceeds from stock options exercised 5.0 $ 8.1 $ 13.7
Tax benefit from stock options exercised $ 0.0 0.0 1.0
Stock Option Plans, 1996 Stock Option Plan, 2000 Employee Stock Option Plan, 2000 Directors Stock Option Plan, 2011 Directors Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 3,475,000    
Duration of award 10 years    
Stock Option Plans, 1996 Stock Option Plan, 2000 Employee Stock Option Plan, 2000 Directors Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 6 months    
Stock Option Plans, 2011 Director Stock Option Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Number of shares available for future grant 18,000    
Stock Option Plans, Independent Insurance Agent Stock Option Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 8,000,000    
Stock Option Plans, All Stock Option Plans For Officers, Directors And Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options exercised in period, total intrinsic value $ 4.0 $ 1.4 $ 5.4