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CONDENSED CONSOLIDATED BALANCE SHEETS (unaudited) (Parenthetical) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Current assets:        
Allowance for doubtful accounts receivable $ 41,598 $ 40,474 $ 40,393 $ 29,381