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Credit Facility (Narrative) (Details) - Credit Facility [Member] - USD ($)
4 Months Ended 8 Months Ended 12 Months Ended
Apr. 18, 2019
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Feb. 25, 2020
Schedule Of Debt [Line Items]          
Credit Facility borrowing capacity   $ 500,000,000 $ 500,000,000    
Expiration of Credit Facility Mar. 31, 2020 Apr. 19, 2024      
Interest rate spread (LIBOR) 0.85% 0.70% 0.70%    
Frequency of commitment fee     quarterly    
Commitment fee percentage 0.08% 0.07% 0.07%    
Outstanding borrowings   $ 0 $ 0    
Reduction in borrowing capacity to amount of letters of credit     $ 15,900,000 $ 16,200,000  
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