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Long-Term Debt (Narrative) (Details)
6 Months Ended
Jan. 24, 2019
USD ($)
Jun. 30, 2021
USD ($)
debtCovenant
Dec. 31, 2020
USD ($)
Revolving Credit Facility | Senior Credit Agreement      
Debt Instrument [Line Items]      
Revolving credit facility, availability amount   $ 360,200,000 $ 309,200,000
Standby Letters of Credit | Senior Credit Agreement      
Debt Instrument [Line Items]      
Letters of credit issued   $ 6,600,000  
Line of Credit | Revolving credit facility      
Debt Instrument [Line Items]      
Current borrowing capacity $ 400,000,000    
Letters of credit issued $ 700,000,000    
Number of debt covenants | debtCovenant   3  
Line of Credit | Revolving credit facility | Minimum      
Debt Instrument [Line Items]      
Revolving credit facility, undrawn commitment fee 0.15%    
Line of Credit | Revolving credit facility | Minimum | London Interbank Offered Rate (LIBOR)      
Debt Instrument [Line Items]      
Revolving credit facility, interest rates margin 1.125%    
Line of Credit | Revolving credit facility | Maximum      
Debt Instrument [Line Items]      
Revolving credit facility, undrawn commitment fee 0.25%    
Line of Credit | Revolving credit facility | Maximum | London Interbank Offered Rate (LIBOR)      
Debt Instrument [Line Items]      
Revolving credit facility, interest rates margin 2.00%    
Medium-term Notes | Revolving credit facility      
Debt Instrument [Line Items]      
Debt instrument, issued value $ 300,000,000