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Debt and Credit Agreements - Narrative (Details) - USD ($)
1 Months Ended
Jul. 31, 2018
Jun. 30, 2018
Apr. 18, 2018
Dec. 31, 2017
Revolving credit facility        
Debt Instrument [Line Items]        
Borrowing base     $ 3,200,000,000.0  
Remaining borrowing capacity   $ 1,700,000,000 $ 1,700,000,000  
Line of credit   $ 0    
6.51% weighted-average senior notes        
Debt Instrument [Line Items]        
Weighted average interest rate   6.51%   6.51%
Subsequent Event | 6.51% weighted-average senior notes        
Debt Instrument [Line Items]        
Repayments of senior notes $ 237,000,000