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STOCKHOLDERS' AND MEMBERS' EQUITY STRUCTURE Treasury Stock (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Class of Stock [Line Items]    
Treasury stock, at cost, 6,050,717 shares outstanding at December 31, 2015 and 5,765,946 shares outstanding at December 31, 2014 $ 160,800 $ 152,585
Class A Common Stock [Member]    
Class of Stock [Line Items]    
Treasury stock, at cost, 6,274,261 shares outstanding at December 31, 2018 and 6,132,884 shares outstanding at December 31, 2017 (in shares) 6,274,261 6,132,884
Repurchase Program [Member] | Class A Common Stock [Member]    
Class of Stock [Line Items]    
Treasury stock, at cost, 6,274,261 shares outstanding at December 31, 2018 and 6,132,884 shares outstanding at December 31, 2017 (in shares) 6,050,717  
Treasury stock, at cost, 6,050,717 shares outstanding at December 31, 2015 and 5,765,946 shares outstanding at December 31, 2014 $ 150,804