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Credit Facility (Narrative) (Details) - Credit Facility - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Nov. 01, 2021
Schedule Of Debt [Line Items]      
Credit Facility borrowing capacity $ 500,000,000    
Interest rate spread 0.75%    
Facility fee percentage 0.10%    
Outstanding borrowings $ 0    
Reduction in borrowing capacity to amount of letters of credit $ 21,200,000 $ 24,300,000  
Minimum      
Schedule Of Debt [Line Items]      
Interest rate spread 0.70%    
Facility fee percentage 0.07%    
Maximum      
Schedule Of Debt [Line Items]      
Interest rate spread 1.35%    
Facility fee percentage 0.25%    
Subsequent Event      
Schedule Of Debt [Line Items]      
Outstanding borrowings     $ 0