Condensed Consolidated Balance Sheets Parenthetical - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts Receivable, Current	$ 21,466	$ 25,549
Common Class A [Member]
Common Stock, Shares, Issued	24,736,009	24,679,127
Common Stock, Shares, Outstanding	24,735,427	24,678,545
Common Class B [Member]
Common Stock, Shares, Issued	5,100,540	5,107,674
Common Stock, Shares, Outstanding	5,099,623	5,106,757
Treasury Class A [Member]
Treasury Stock, Shares	582	582
Treasury Class B [Member]
Treasury Stock, Shares	917	917