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Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Allowance for doubtful accounts $ 1,781,075 $ 1,623,408
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 4,377,227 3,032,740
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 15,221,715 15,222,738
Common stock, shares outstanding 10,844,488 12,189,998
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