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Equity-Based Compensation Plans - Summary of Share-based Compensation Arrangements by Share-based Payment Award (Detail) - Profit Interest Units [Member] - $ / shares
9 Months Ended 12 Months Ended
Sep. 26, 2021
Sep. 27, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Outstanding PIUs, Granted 8,445 5,932      
Outstanding PIUs, Ending Balance 36,045 4,507      
Weighted Average Grant Date Fair Value, Ending Balance $ 0.64 $ 0.27      
Incentive Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Outstanding PIUs, Beginning Balance 35,570 31,831 31,831 27,925
Outstanding PIUs, Granted     5,932 3,906 27,925
Outstanding PIUs, Forfeited     (2,193)    
Outstanding PIUs, Ending Balance     35,570 31,831 27,925
Weighted Average Grant Date Fair Value, Beginning Balance $ 0.27 $ 0.27 $ 0.27 $ 0.27
Weighted Average Grant Date Fair Value, Granted     0.28 0.25 0.27
Weighted Average Grant Date Fair Value, Forfeited     0.27    
Weighted Average Grant Date Fair Value, Ending Balance     $ 0.27 $ 0.27 $ 0.27