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Share-Based Compensation - Weighted average assumptions used in valuing options (Details) - USD ($)
12 Months Ended 90 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2009
Employee Stock Purchase Plan        
Additional disclosures        
Compensation expense $ 75,000 $ 82,000 $ 43,000  
Employee Stock Purchase Plan        
Number of shares of common stock registered in connection with the establishment of an Employee Stock Purchase Plan 363,825      
Requisite age of employees to be eligible to participate in the plan 21 years      
Requisite service period to be eligible to participate in the plan 6 months      
Discount rate for eligible employees to purchase company stock (as a percent) 5.00%     15.00%
Minimum | Employee Stock Purchase Plan        
Employee Stock Purchase Plan        
Shares available for issuance under the plan 50,000      
Maximum | Employee Stock Purchase Plan        
Employee Stock Purchase Plan        
Estimated subscription date fair value (as a percent) 95.00%      
Stock Options        
Weighted-average assumptions        
Dividend yield (as a percent)   1.46% 1.40%  
Expected life   8 years 6 months 8 years 6 months  
Expected volatility (as a percent)   28.00% 37.20%  
Risk-free interest rate (as a percent)   2.54% 2.43%  
Additional disclosures        
Total unrecognized compensation cost related to non vested stock option grants $ 720,000      
Weighted-average period over which unrecognized compensation cost is expected to be recognized 9 months 3 days      
Total fair value of shares vested during the period $ 799,000 $ 700,000 $ 578,000  
Compensation expense $ 641,000 751,000 784,000  
Restricted Stock        
Additional disclosures        
Weighted-average period over which unrecognized compensation cost is expected to be recognized 1 year 3 months      
Compensation expense $ 1,700,000 $ 2,500,000 $ 2,500,000  
Restricted Stock Activity and RSUs        
Nonvested at the beginning of the period (in shares) 104,419      
Granted (in shares) 8,934 7,836 26,053  
Vested (in shares) (40,858)      
Forfeited (in shares) (3,045)      
Nonvested at the end of the period (in shares) 69,450 104,419    
Weighted-Average Grant-Date Fair Value        
Nonvested at the beginning of the period (in dollars per share) $ 62.45      
Granted (in dollars per share) 73.34 $ 87.37 $ 89.11  
Vested (in dollars per share) 70.06      
Forfeited (in dollars per share) 72.01      
Nonvested at the end of the period (in dollars per share) $ 58.96 $ 62.45    
Additional disclosures        
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted $ 1,100,000      
Total fair value of shares vested during the period 3,000,000.0 $ 2,500,000 $ 3,100,000  
Employee Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 3,000,000.0 2,500,000 3,100,000  
Restricted Stock | Employees | Minimum        
Additional disclosures        
Vesting period 4 years      
Restricted Stock | Non-employee directors | Maximum        
Additional disclosures        
Vesting period 12 months      
Restricted Stock Units        
Additional disclosures        
Weighted-average period over which unrecognized compensation cost is expected to be recognized 1 year 9 months 3 days      
Compensation expense $ 6,400,000 $ 5,500,000 $ 3,600,000  
Restricted Stock Activity and RSUs        
Nonvested at the beginning of the period (in shares) 200,540      
Granted (in shares) 159,521 113,270 77,301  
Vested (in shares) (31,723)      
Forfeited (in shares) (3,737)      
Nonvested at the end of the period (in shares) 324,601 200,540    
Weighted-Average Grant-Date Fair Value        
Nonvested at the beginning of the period (in dollars per share) $ 85.10      
Granted (in dollars per share) 68.27      
Vested (in dollars per share) 89.40      
Forfeited (in dollars per share) 82.18      
Nonvested at the end of the period (in dollars per share) $ 76.44 $ 85.10    
Additional disclosures        
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted $ 10,500,000      
Total fair value of shares vested during the period $ 5,800,000 $ 5,700,000 $ 5,000,000.0  
Target RSU award level (as a percent) 84.30%      
Employee Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 5,800,000 $ 5,700,000 $ 5,000,000.0  
Performance based restricted stock units        
Additional disclosures        
Performance period 3 years      
Other Performance based restricted stock units        
Additional disclosures        
Performance period 1 year      
Timed based restricted stock units        
Additional disclosures        
Performance period 4 years