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Retirement and Share-based Compensation Plans (Incentive Plans) (Details) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
2016 Employee Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 2,500,000    
Number of shares available for future grant 2,101,954    
2009 Employee Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 2,500,000    
Duration of award 10 years    
Award vesting period 3 years    
2009 Employee Incentive Plan [Member] | March 2010 Option Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Duration of award 6 years    
Employee Incentive Plans [Member] | Restricted Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Equity instruments granted 84,476 208,565 60,947
Duration used to measure performance and threshold goals 3 years    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Threshold Goals, Vesting Percentage [Member] | 2017 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 50.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Threshold Goals, Vesting Percentage [Member] | 2016 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 50.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Threshold Goals, Vesting Percentage [Member] | 2015 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 50.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Target Performance Goals, Vesting Percentage [Member] | 2017 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 100.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Target Performance Goals, Vesting Percentage [Member] | 2016 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 100.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Target Performance Goals, Vesting Percentage [Member] | 2015 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 100.00%    
Employee Incentive Plans [Member] | Restricted Stock Units [Member] | Maximum Performance Goals, Vesting Percentage [Member] | 2017 Employee Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting rights, percentage 150.00%    
Employee Incentive Plans [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Shares issued in period 39,826 43,373 25,784
Participant age for full vesting 65 years    
Requisite service period for full vesting 10 years    
Employee Incentive Plans [Member] | Restricted Stock [Member] | 2016 Revised Grant [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued in period   21,806  
Employee Incentive Plans [Member] | Restricted Stock [Member] | Share-based Compensation Award, Tranche One [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 1 year    
Shares issued in period 6,727    
2013 Director Equity And Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized 250,000    
Number of shares available for future grant 83,500    
2013 Director Equity And Incentive Plan [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares issued in period 33,000 47,500 22,000
Maximum [Member] | 2013 Director Equity And Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 1 year