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Condensed Consolidated Balance Sheets Parenthetical - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Allowance for Doubtful Accounts Receivable, Current $ 28,443 $ 25,549
Common Class A [Member]    
Common Stock, Shares, Issued 24,701,515 24,679,127
Common Stock, Shares, Outstanding 24,700,933 24,678,545
Common Class B [Member]    
Common Stock, Shares, Issued 5,104,674 5,107,674
Common Stock, Shares, Outstanding 5,103,757 5,106,757
Treasury Class A [Member]    
Treasury Stock, Shares 582 582
Treasury Class B [Member]    
Treasury Stock, Shares 917 917