Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable, trade, allowance for doubtful accounts	$ 12,949	$ 9,691
Property and equipment, accumulated depreciation	$ 290,229	$ 259,919
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	45,000,000	45,000,000
Preferred stock, shares issued	0	0
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	63,235,291	62,721,145
Common stock, shares outstanding	45,025,364	49,601,184
Treasury stock, shares	18,209,927	13,119,961
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