CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) vote $ / shares shares	Dec. 31, 2020 USD ($) vote $ / shares shares
Accounts receivable - trade, allowance for credit losses | $	$ 3,057	$ 2,333
Accumulated depreciation and amortization | $	$ 952,032	$ 900,882
Class A Common Stock
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	50,410,000	50,101,000
Common stock, shares outstanding (in shares)	50,410,000	50,101,000
Class B Common Stock
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,000,000	1,000,000
Common stock, shares issued (in shares)	988,000	988,000
Common stock, shares outstanding (in shares)	988,000	988,000
Votes per share held (in votes) | vote	10	10