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Employee Benefit Plans, Restricted Stock Plan (Details) - Restricted Stock [Member] - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares authorized (in shares) 150,000  
Number of shares available for grant (in shares) 141,408  
Unvested Shares [Roll Forward]    
Outstanding, beginning of period (in shares) 8,783  
Granted (in shares) 0  
Forfeited (in shares) 0  
Vested (in shares) (92)  
Outstanding, end of period (in shares) 8,691  
Weighted Average Market Price [Roll Forward]    
Outstanding, beginning of period (in dollars per share) $ 51.20  
Granted (in dollars per share) 0  
Forfeited (in dollars per share) 0  
Vested (in dollars per share) 52.05  
Outstanding, end of period (in dollars per share) $ 51.19  
Additional General Disclosures [Abstract]    
Share-based compensation expense $ 56 $ 50
Compensation cost related to nonvested awards that has not yet been recognized $ 445  
Period over which compensation cost is expected to be recognized 3 years