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Debt (2018 Revolving Credit Facility - Narrative) (Details) - USD ($)
1 Months Ended 6 Months Ended
Apr. 30, 2018
Jun. 30, 2020
Apr. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]        
Subsidiary ownership   100.00%    
Letters of credit   $ 209,000,000    
Debt instrument   2,457,000,000   $ 2,262,000,000
Long-term debt | 2018 revolving credit facility due April 2024        
Debt Instrument [Line Items]        
Maximum borrowing capacity $ 3,500,000,000      
Current borrowing base     $ 1,800,000,000  
Aggregate commitment     $ 1,800,000,000  
Limit on securing indebtedness $ 2,000,000,000.0      
Percentage of consolidated net tangible assets 25.00%      
Minimum interest coverage ratio 1.00      
Long-term debt | 2018 revolving credit facility due April 2024 | Maximum        
Debt Instrument [Line Items]        
Leverage ratio, percentage of credit limit 10.00%      
Leverage ratio, amount of credit limit $ 150,000,000      
Long-term debt | 2018 revolving credit facility due April 2024 | Eurodollar | Minimum        
Debt Instrument [Line Items]        
Basis points 1.50%      
Long-term debt | 2018 revolving credit facility due April 2024 | Eurodollar | Maximum        
Debt Instrument [Line Items]        
Basis points 2.50%      
Long-term debt | 2018 revolving credit facility due April 2024 | Base Rate | Minimum        
Debt Instrument [Line Items]        
Basis points 0.50%      
Long-term debt | 2018 revolving credit facility due April 2024 | Base Rate | Maximum        
Debt Instrument [Line Items]        
Basis points 1.50%      
Line of Credit | 2018 Revolving Credit Facility        
Debt Instrument [Line Items]        
Debt instrument   $ 336,000,000    
June 30, 2018 Through March 31, 2019 | Long-term debt | 2018 revolving credit facility due April 2024 | Maximum        
Debt Instrument [Line Items]        
Leverage ratio 4.50      
June 30, 2019 Through March 31, 2020 | Long-term debt | 2018 revolving credit facility due April 2024 | Maximum        
Debt Instrument [Line Items]        
Leverage ratio 4.25      
After June 30, 2020 | Long-term debt | 2018 revolving credit facility due April 2024 | Maximum        
Debt Instrument [Line Items]        
Leverage ratio 4.00