Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable, trade, allowance for doubtful accounts	$ 10,788	$ 9,691
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	45,000,000	45,000,000
Preferred stock, shares issued	0	0
Treasury stock, shares	18,506,512	13,119,961
Common Class A [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	360,000,000	360,000,000
Common Stock, Shares, Issued	63,255,117	62,721,145
Common Stock, Shares, Outstanding	44,748,605	49,601,184
Common Class B [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	90,000,000	90,000,000
Common Stock, Shares, Issued	11,484,408	11,484,408
Common Stock, Shares, Outstanding	11,484,408	11,484,408