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Debt (Narrative) (Details) - USD ($)
Feb. 21, 2020
Feb. 24, 2020
Dec. 31, 2019
Jun. 30, 2019
Debt Instrument [Line Items]        
Credit facility, borrowing base     $ 220,000,000.0  
Letters of Credit Outstanding, Amount     102,500,000  
Letter of Credit [Member]        
Debt Instrument [Line Items]        
Credit facility, borrowing capacity     150,000,000.0 $ 100,000,000.0
Unsecured Senior Notes Due 2024 [Member]        
Debt Instrument [Line Items]        
Senior Notes     $ 500,000,000  
Stated interest rate on unsecured senior notes     7.75%  
Principal amount     $ 500,000,000  
Unsecured Senior Notes Due 2021 [Member]        
Debt Instrument [Line Items]        
Senior Notes     $ 800,000,000.0  
Stated interest rate on unsecured senior notes     7.125%  
Unsecured Senior Notes Due 2021 [Member] | Subsequent Event [Member]        
Debt Instrument [Line Items]        
Senior Notes   $ 182,100,000    
New Notes [Member]        
Debt Instrument [Line Items]        
Stated interest rate on unsecured senior notes     7.125%  
New Notes [Member] | Subsequent Event [Member]        
Debt Instrument [Line Items]        
Principal amount $ 617,900,000      
Percentage of new principal as a percentage of original notes 77.24%