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Stock Purchase and Option Plans (Details) (USD $)
 1 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
May 31, 2012
May 31, 2011
May 31, 2010
Jun. 30, 2009
Dec. 31, 2012
Stock Options [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Outside Director [Member]
Dec. 31, 2011
Restricted Stock Units (RSUs) [Member]
Outside Director [Member]
Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
Outside Director [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Class B [Member]
Outside Director [Member]
Dec. 31, 2011
Restricted Stock Units (RSUs) [Member]
Class B [Member]
Outside Director [Member]
Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
Class B [Member]
Outside Director [Member]
Feb. 04, 2013
Restricted Stock Units (RSUs) [Member]
Class B [Member]
Executives [Member]
Feb. 09, 2011
Restricted Stock Units (RSUs) [Member]
Class B [Member]
Executives [Member]
Dec. 31, 2012
Stock Purchase Plan [Member]
Dec. 31, 2012
Stock Purchase Plan [Member]
Class A [Member]
Dec. 31, 2012
Stock Purchase Plan [Member]
Class B [Member]
Dec. 31, 2012
Director Option Plan [Member]
Class B [Member]
Non-Employee Director [Member]
Dec. 31, 2012
Director Option Plan [Member]
Stock Options [Member]
Non-Employee Director [Member]
Dec. 31, 2011
Director Option Plan [Member]
Stock Options [Member]
Non-Employee Director [Member]
Dec. 31, 2010
Director Option Plan [Member]
Stock Options [Member]
Non-Employee Director [Member]
Dec. 31, 2012
Employee Option Plan [Member]
Class B [Member]
Employees [Member]
Dec. 31, 2012
Employee Option Plan [Member]
Stock Options [Member]
Employees [Member]
Dec. 31, 2011
Employee Option Plan [Member]
Stock Options [Member]
Employees [Member]
Dec. 31, 2010
Employee Option Plan [Member]
Stock Options [Member]
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                                  
Purchase price of shares obligated to be repurchased, percentage of book value (in hundredths)                             90.00%                    
Number of shares repurchased under the plan (in shares)                             0                    
Number of outstanding shares eligible for repurchase (in shares)                               124,099 380,458                
Number of share-based compensation plan         2 1                                      
Shares of common stock reserved for granting of discounted and market value option (in shares)                                   300,000       1,125,000      
Common stock available for future grants (in shares)                                   167,000       259,000      
Options granted (in shares)                                     0   0   0    
Discounted options outstanding (in shares)                                     0 0 0   0    
Discounted option exercised (in shares)                                     0 0 0   0 0 0
Compensation cost charged against income             $ 440,000 $ 440,000 $ 440,000                   $ 0   $ 0   $ 0    
Shares issued (in shares)                         52,389 14,473                      
Value of each share issued (in dollars per share) $ 21.87 $ 22.50 $ 24.78 $ 21.05                 $ 23.69 $ 23.39                      
Total value of shares issued                   $ 440,000 $ 440,000 $ 440,000 $ 1,241,000 $ 339,000                      
Summary of stock Issuances [Abstract]                                                  
Shares granted (in shares) 20,119 19,558 17,754 20,900                                          
Vesting Date May 08, 2013 May 10, 2012 May 04, 2011 May 05, 2010                                          
Period 7/1/2012 - 6/30/2013 7/1/2011 - 6/30/2012 7/1/2010 - 6/30/2011 7/1/2009 - 6/30/2010                                          
Period restricted stocks will vest ratably from date of grant           3 years