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Long-term Debt - Narrative - Senior Credit Facility (Details) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Debt Instrument [Line Items]    
Outstanding debt balance $ 434,700,000 $ 608,500,000
Line of credit | Letter of credit    
Debt Instrument [Line Items]    
Letters of credit outstanding $ 27,600,000  
Senior Credit Facility, Due 2023 | Secured debt    
Debt Instrument [Line Items]    
Maximum leverage ratio increases for two consecutive quarters 0.50  
Senior Credit Facility, Due 2023 | Secured debt | Minimum    
Debt Instrument [Line Items]    
Basis spread on variable rate on debt instrument 1.75%  
Required coverage ratio 3.00  
Senior Credit Facility, Due 2023 | Secured debt | Maximum    
Debt Instrument [Line Items]    
Basis spread on variable rate on debt instrument 2.50%  
Leverage ratio on pro forma basis 3.25  
Senior Credit Facility, Due 2023 | Secured debt | Maximum | October 1, 2020 - September 30, 2021    
Debt Instrument [Line Items]    
Required leverage ratio 3.75  
Senior Credit Facility, Due 2023 | Secured debt | Maximum | October 1, 2021, therafter    
Debt Instrument [Line Items]    
Required leverage ratio 3.50  
Senior Credit Facility, Due 2023 | Secured debt | Revolving Credit Facility    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 650,000,000  
Outstanding debt balance $ 0  
Senior Credit Facility, Due 2023 | Secured debt | Revolving Credit Facility | Minimum    
Debt Instrument [Line Items]    
Commitment fee percentage 0.30%  
Senior Credit Facility, Due 2023 | Secured debt | Revolving Credit Facility | Maximum    
Debt Instrument [Line Items]    
Commitment fee percentage 0.45%  
Senior Credit Facility, Due 2023 | Secured debt | Term Loan    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 350,000,000  
Senior Credit Facility, Due 2023 | Secured debt | Term Loan | December 31, 2020 - December 30, 2022    
Debt Instrument [Line Items]    
Annual principal payment as a percent   7.50%
Senior Credit Facility, Due 2023 | Secured debt | Term Loan | December 31, 2022 - maturity    
Debt Instrument [Line Items]    
Annual principal payment as a percent 10.00%  
Senior Credit Facility, Due 2023 | Secured debt | Letter of credit | Minimum    
Debt Instrument [Line Items]    
Unused capacity commitment fee percentage 1.75%  
Senior Credit Facility, Due 2023 | Secured debt | Letter of credit | Maximum    
Debt Instrument [Line Items]    
Unused capacity commitment fee percentage 2.50%