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CONSOLIDATED BALANCE SHEETS (Parenthetical)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2021
USD ($)
vote
$ / shares
shares
Dec. 31, 2020
USD ($)
vote
$ / shares
shares
Accounts receivable - trade, allowance for doubtful accounts | $ $ 2,095 $ 2,333
Accumulated depreciation and amortization | $ $ 916,529 $ 900,882
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 50,374,000 50,101,000
Common stock, shares outstanding 50,374,000 50,101,000
Class B Common Stock    
Common stock, par value (in dollars per share) | $ / shares $ 0.01 $ 0.01
Common stock, shares authorized 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