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Stock Purchase and Option Plans (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Feb. 08, 2017
USD ($)
$ / shares
shares
May 10, 2016
$ / shares
shares
Feb. 05, 2016
USD ($)
$ / shares
shares
May 12, 2015
$ / shares
shares
May 08, 2014
$ / shares
shares
Dec. 31, 2016
USD ($)
Director
Plan
shares
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Restricted Stock Units (RSUs) [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of share based compensation plans | Plan           1    
Restricted Stock Units (RSUs) [Member] | Outside Director [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Compensation cost charged against income | $           $ 460 $ 460 $ 440
Restricted Stock Units (RSUs) [Member] | Class B [Member] | Outside Director [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Total value of shares issued | $           $ 440 $ 480 $ 440
Summary of stock Issuances [Abstract]                
Number of shares issued (in shares)   17,677   21,252 17,237      
Vesting Date   May 10, 2017   May 12, 2016 May 08, 2015      
Period   7/1/2016 - 6/30/2017   7/1/2015 - 6/30/2016 7/1/2014 - 6/30/2015      
Value of each share issued on grant date (in dollars per share) | $ / shares   $ 24.89   $ 22.59 $ 25.53      
Restricted Stock Units (RSUs) [Member] | Class B [Member] | Executives [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares issued (in shares)     47,333          
Total value of shares issued | $     $ 1,103          
Summary of stock Issuances [Abstract]                
Value of each share issued on grant date (in dollars per share) | $ / shares     $ 23.30          
Period restricted stocks will vest ratably from date of grant     3 years          
Restricted Stock Units (RSUs) [Member] | Class B [Member] | Executives [Member] | Subsequent Event [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares issued (in shares) 20,181              
Total value of shares issued | $ $ 480              
Summary of stock Issuances [Abstract]                
Value of each share issued on grant date (in dollars per share) | $ / shares $ 23.80              
Period restricted stocks will vest ratably from date of grant 3 years              
1981 Stock Purchase Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Purchase price of shares obligated to be repurchased, percentage of book value           90.00%    
Number of outside directors | Director           2    
1981 Stock Purchase Plan [Member] | Class A [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of outstanding shares eligible for repurchase (in shares)           60,938    
1981 Stock Purchase Plan [Member] | Class B [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of outstanding shares eligible for repurchase (in shares)           240,938