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Stock-Based Compensation (Details) (USD $)
12 Months Ended
Dec. 31, 2012
plan
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2005
Stock-Based Compensation        
Number of stock-based compensation plans 1      
Number of shares reserved under the plan       735,000
Number of additional shares authorized under the plan     700,000  
Number of shares available for future awards 580,467      
Stock-based compensation        
Stock-based compensation expense before income taxes $ 1,481,000 $ 1,165,000 $ 952,000  
Deferred tax benefit 518,000 408,000 333,000  
Reduction of net income 963,000 757,000 619,000  
Unrecognized stock-based compensation expense 3,149,000      
Period of recognition of stock-based compensation expense 5 years      
Amount received from exercise of options $ 937,000 $ 692,000 $ 785,000  
Options and stock appreciation rights (SARs)
       
Stock-based compensation        
Award vesting limit per year (as a percent) 20.00%      
Period after grant date after which the award expires 10 years      
Assumptions used in SAR/Option valuations        
Dividend yield (as a percent) 2.52% 2.48% 2.18%  
Expected volatility (as a percent) 22.04% 22.64% 23.87%  
Risk free interest rate (as a percent) 1.44% 2.90% 3.57%  
Forfeitures (as a percent) 4.20% 6.07% 5.96%  
Expected life of awards 7 years 7 months 6 days 7 years 6 months 7 years 7 months 6 days  
Restricted stock | Minimum
       
Stock-based compensation        
Award vesting period 3 years      
Restricted stock | Maximum
       
Stock-based compensation        
Award vesting period 5 years