CONSOLIDATED BALANCE SHEETS (Parenthetical) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) vote $ / shares shares	Dec. 31, 2019 USD ($) vote $ / shares shares
Accounts receivable - trade, allowance for doubtful accounts | $	$ 2,681	$ 1,468
Accumulated depreciation and amortization | $	$ 890,386	$ 844,874
Class A Common Stock
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	47,384,000	46,803,000
Common stock, shares outstanding	47,384,000	46,803,000
Class B Common Stock
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Common stock, shares authorized	1,000,000	1,000,000
Common stock, shares issued	988,000	988,000
Common stock, shares outstanding	988,000	988,000
Votes per share held | vote	10	10