Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable, trade, allowance for doubtful accounts	$ 11,234	$ 5,733
Accumulated depreciation and amortization	$ 261,082	$ 347,302
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	45,000,000	45,000,000
Preferred stock, shares issued (in shares)	0	0
Treasury stock (in shares)	34,593,000	19,808,000
Common Class A [Member]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	360,000,000	360,000,000
Common stock, shares, issued (in shares)	64,568,000	63,886,000
Common stock, shares, outstanding (in shares)	29,975,000	44,078,000
Common Class B [Member]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	90,000,000	90,000,000
Common stock, shares, issued (in shares)	11,484,000	11,484,000
Common stock, shares, outstanding (in shares)	11,484,000	11,484,000