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Condensed Consolidated Balance Sheets Parenthetical - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Allowance for Doubtful Accounts Receivable, Current $ 26,559 $ 25,549
Common Class A [Member]    
Common Stock, Shares, Issued 24,860,159 24,679,127
Common Stock, Shares, Outstanding 24,859,577 24,678,545
Common Class B [Member]    
Common Stock, Shares, Issued 5,103,544 5,107,674
Common Stock, Shares, Outstanding 5,102,627 5,106,757
Treasury Class A [Member]    
Treasury Stock, Shares 582 582
Treasury Class B [Member]    
Treasury Stock, Shares 917 917