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Equity Plans Equity Plans (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
USD ($)
purchase_interval
shares
Dec. 31, 2014
USD ($)
shares
Dec. 31, 2013
USD ($)
shares
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income $ 1,603 $ 1,031 $ 912
Proceeds from the exercise of stock options and similar instruments 895 1,917 1,004
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net $ 634 $ 462 $ 28
Employee Stock Purchase Plan [Abstract]      
Number of purchases intervals under the plan since inception | purchase_interval 19    
Shares issued during the period under the plan | shares 33,318 35,682 30,869
Cash received from employees toward purchases under the plan $ 916 $ 854 $ 796
Restricted Stock and Deferred Restricted Stock [Member]      
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income 1,445 883 784
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net 634 $ 249 $ 97
Employee Stock Purchase Plan [Abstract]      
Unrecognized compensation cost $ 1,231    
Deferred Restricted Stock Awards [Member]      
Dividend Equivalent Rights [Abstract]      
Cumulative dividend equivalent rights converted to deferred restricted stock awards | shares 70,691 66,458 61,733
Employee Stock Option [Member]      
Summary of Activity under the Plan [Abstract]      
Proceeds from the exercise of stock options and similar instruments $ 0 $ 1,044 $ 198
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net 0 213 (69)
Dividend Equivalent Rights [Member]      
Dividend Equivalent Rights [Abstract]      
Liability for dividend equivalent rights $ 114 $ 122 $ 128
Incentive Plan [Member]      
Option Indexed to Issuer's Equity [Line Items]      
Number of shares reserved for issuance under the plan | shares 1,800,000    
Maximum number of shares per employee | shares 600,000    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued Pursuant To The Plan | shares 516,956 429,352 341,914
Shares issuable under the plan upon exercise of outstanding awards | shares 245,976 296,831 307,919
Number of shares available for grant | shares 1,037,068    
Employee Stock [Member]      
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income $ 158 $ 148 $ 128
Proceeds from the exercise of stock options and similar instruments $ 895 839 722
Employee Stock Purchase Plan [Abstract]      
Purchase price of common stock to employees under the plan, percent 85.00%    
Maximum amount of base salary employees can designate for share purchase under the plan 10.00%    
Expenses recorded under the plan $ 162 151 140
Unrecognized compensation cost $ 64    
Employee Stock [Member] | Common Stock [Member]      
Option Indexed to Issuer's Equity [Line Items]      
Number of shares reserved for issuance under the plan | shares 400,000    
Dividend Reinvestment and Stock Purchase Plan [Member]      
Option Indexed to Issuer's Equity [Line Items]      
Number of shares reserved for issuance under the plan | shares 3,000,000    
Summary of Activity under the Plan [Abstract]      
Proceeds from the exercise of stock options and similar instruments $ 0 $ 34 $ 84
Employee Stock Purchase Plan [Abstract]      
Shares issued under the plan | shares   829 3,487