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Debt - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Feb. 12, 2019
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Line of Credit Facility            
Line of credit maximum borrowing amount   $ 300,000,000   $ 300,000,000   $ 300,000,000
Debt amortization expense   $ 42,000 $ 42,000 $ 126,000 $ 126,000  
Revolving Credit Facility Due 2024            
Line of Credit Facility            
Line of credit maximum borrowing amount $ 300,000,000          
Line of credit facility contingent increase to maximum borrowing capacity $ 150,000,000          
Commitment fee percentage per annum       0.20%   0.23%
Revolving Credit Facility Due 2024 | Minimum            
Line of Credit Facility            
Commitment fee percentage per annum 0.20%          
Revolving Credit Facility Due 2024 | Maximum            
Line of Credit Facility            
Commitment fee percentage per annum 0.30%          
Revolving Credit Facility Due 2024 Swingline Sublimit            
Line of Credit Facility            
Line of credit maximum borrowing amount $ 15,000,000          
Revolving Credit Facility Due 2024 Letter Of Credit Sublimit            
Line of Credit Facility            
Line of credit maximum borrowing amount $ 10,000,000          
Line of Credit | Revolving Credit Facility Due 2024            
Line of Credit Facility            
Debt instrument, term 5 years