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NET INCOME PER COMMON SHARE AND EQUITY PER COMMON SHARE (Details) - $ / shares
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Shares used in computing basic and diluted net income per share [Abstract]          
Weighted average common shares outstanding during the period for Basic computation (in shares) 9,244,000 9,191,000 9,235,000 9,164,000  
Dilutive effect of employee stock options and awards (in shares) 55,000 95,000 55,000 98,000  
Weighted average common shares outstanding during the period for Diluted computation (in shares) 9,299,000 9,286,000 9,290,000 9,262,000  
Class A Stock [Member]          
Common Stock [Abstract]          
Common stock, shares authorized (in shares) 15,000,000   15,000,000    
Common stock, shares issued (in shares) 8,364,231 8,311,100 8,364,231 8,311,100  
Common stock, shares outstanding (in shares) 8,364,231 8,311,100 8,364,231 8,311,100  
Common stock, par value (in dollars per share) $ 1.00   $ 1.00    
Common stock, shares issued during period (in shares) 4,493 5,027 30,777 65,067  
Earnings Per Share [Abstract]          
Equity per common share (in dollars per share)     $ 16.25   $ 15.98
Class B Stock [Member]          
Common Stock [Abstract]          
Common stock, shares authorized (in shares) 1,040,000   1,040,000    
Common stock, shares issued (in shares) 881,452 881,452 881,452 881,452  
Common stock, shares outstanding (in shares) 881,452 881,452 881,452 881,452  
Common stock, par value (in dollars per share) $ 1.00   $ 1.00    
Stock Option [Member]          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]          
Shares excluded from calculations of diluted net income per share (in shares) 0 0 0 0  
Restricted Stock [Member]          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract]          
Shares excluded from calculations of diluted net income per share (in shares) 0 0 4,108 4,240