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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Lettersofcredit
Dec. 31, 2020
USD ($)
Lettersofcredit
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
Loss Contingencies [Line Items]        
Number of letters of credit | Lettersofcredit 2 3    
Letters of credit as additional security for lease guarantees $ 0.6 $ 0.7    
Decrease in revenue   $ 22.5    
Decrease in revenue percentage   7.20%    
Revenue   $ 289.5 $ 312.1  
Percentage of taxable income offset by NOL carryforwards and carrybacks 100.00% 100.00%    
Net operating loss carrybacks, Period 5 years 5 years    
Tax-deductible interest, base percentage   50.00% 50.00% 30.00%
Salary Exchange Program [Member] | Employee Stock Options [Member]        
Loss Contingencies [Line Items]        
Options granted | shares   417,673    
Salary Exchange Program [Member] | Restricted Stock Units [Member]        
Loss Contingencies [Line Items]        
Granted | shares   211,207    
Minimum [Member]        
Loss Contingencies [Line Items]        
Tax-deductible interest, base percentage   30.00%    
Minimum [Member] | Salary Exchange Program [Member]        
Loss Contingencies [Line Items]        
Percentage of salary exchange program for award   2.00%    
Maximum [Member]        
Loss Contingencies [Line Items]        
Tax-deductible interest, base percentage   50.00%    
Maximum [Member] | Salary Exchange Program [Member]        
Loss Contingencies [Line Items]        
Percentage of salary exchange program for award   20.00%