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Equity Plans Equity Plans (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
purchase_interval
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income $ 2,643 $ 1,691 $ 1,603
Proceeds from the exercise of stock options and similar instruments 1,215 954 895
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net     $ 634
Excess Tax Benefit from Share-based Compensation, Financing Activities $ 0 $ 203  
Employee Stock Purchase Plan [Abstract]      
Number of purchases intervals under the plan since inception | purchase_interval 17    
Shares issued during the period under the plan | shares 27,743 30,214 33,318
Cash received from employees toward purchases under the plan $ 1,282 $ 1,060 $ 916
Restricted Stock and Deferred Restricted Stock [Member]      
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income 2,429 1,523 1,445
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net     $ 634
Excess Tax Benefit from Share-based Compensation, Financing Activities 0 $ 203  
Employee Stock Purchase Plan [Abstract]      
Unrecognized compensation cost $ 1,569    
Deferred Restricted Stock Awards [Member]      
Dividend Equivalent Rights [Abstract]      
Cumulative dividend equivalent rights converted to deferred restricted stock awards | shares 77,034 74,403 70,691
Dividend Equivalent Rights [Member]      
Dividend Equivalent Rights [Abstract]      
Liability for dividend equivalent rights $ 139 $ 114 $ 114
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 628,546 576,074 516,956
Shares issuable under the plan upon exercise of outstanding awards | shares 228,885 229,972 245,976
Number of shares available for grant | shares 942,569    
Employee Stock Purchase Plan [Member]      
Summary of Activity under the Plan [Abstract]      
Compensation costs charged to income $ 214 $ 168 $ 158
Proceeds from the exercise of stock options and similar instruments $ 1,215 954 895
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 $ 229 $ 185 $ 162
Unrecognized compensation cost $ 99    
Common Stock [Member] | Employee Stock Purchase Plan [Member]      
Option Indexed to Issuer's Equity [Line Items]      
Number of shares reserved for issuance under the plan | shares 400,000