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Debt - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Apr. 30, 2019
Dec. 31, 2018
Jul. 31, 2017
Debt Instrument [Line Items]          
Debt issuance costs for the 2026 Senior Unsecured Notes $ 8,053     $ 8,214  
Amortization of debt issuance costs 1,255 $ 1,223      
Interest Expense          
Debt Instrument [Line Items]          
Amortization of debt issuance costs 1,000 $ 1,000      
RBL Facility          
Debt Instrument [Line Items]          
Debt issuance costs for the RBL Facility 15,000     16,000  
RBL Facility | Line of credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity         $ 1,500,000
Available borrowing capacity 391,000        
Letters of credit outstanding 9,000     7,000  
2026 Senior Unsecured Notes | Medium-term notes          
Debt Instrument [Line Items]          
Fair value of debt $ 399,000     $ 368,000  
Subsequent Event | RBL Facility | Line of credit          
Debt Instrument [Line Items]          
Borrowing base     $ 750,000    
Elected commitment feature to increase borrowing base     $ 400,000