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Long-term Debt - Summary of Scheduled Maturities of Long-term Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Maturities of Long-term Debt [Abstract]      
2021   $ 6,700  
2022   6,700  
2023   6,700  
2024   6,700  
2025   6,700  
Thereafter   778,150  
Total long-term debt $ 564,724 $ 811,650 $ 551,941