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Consolidated Balance Sheets (Parenthetical) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Allowance for doubtful accounts $ 1,623,408 $ 1,537,353
Preferred stock, par value $ 0.001 $ 0.001
Preferred stock, shares authorized 10,000,000 10,000,000
Preferred stock, shares issued 0 0
Treasury stock, Class A common stock shares 3,032,740 2,937,987
Class A Common Stock [Member]    
Common stock, par value $ 0.001 $ 0.001
Common stock, shares authorized 150,000,000 150,000,000
Common stock, shares issued 15,222,738 15,112,529
Common stock, shares outstanding 12,189,998 12,174,542
Class B Common Stock [Member]    
Common stock, par value $ 0.001 $ 0.001
Common stock, shares authorized 75,000,000 75,000,000
Common stock, shares issued 16,662,743 16,662,743
Common stock, shares outstanding 16,662,743 16,662,743