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Credit Facility (Narrative) (Details) - Credit Facility [Member] - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Aug. 05, 2020
Schedule Of Debt [Line Items]      
Credit Facility borrowing capacity $ 500,000,000    
Expiration of Credit Facility Apr. 19, 2024    
Interest rate spread (LIBOR) 0.70%    
Frequency of commitment fee quarterly    
Commitment fee percentage 0.07%    
Outstanding borrowings $ 0    
Reduction in borrowing capacity to amount of letters of credit $ 19,300,000 $ 15,900,000  
Minimum [Member]      
Schedule Of Debt [Line Items]      
Interest rate spread (LIBOR) 0.70%    
Commitment fee percentage 0.07%    
Maximum [Member]      
Schedule Of Debt [Line Items]      
Interest rate spread (LIBOR) 1.35%    
Commitment fee percentage 0.25%    
Subsequent Event [Member]      
Schedule Of Debt [Line Items]      
Outstanding borrowings     $ 0