Annual report pursuant to Section 13 and 15(d)

Borrowings (Senior Secured Revolving Credit Facility) (Details)

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Borrowings (Senior Secured Revolving Credit Facility) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 20, 2019
May 01, 2019
May 25, 2017
May 19, 2017
Oct. 03, 2016
Line of Credit Facility [Line Items]                
Loss on extinguishment of debt $ 4,881,000 $ 0 $ 0          
Borrowings outstanding 3,185,000,000 1,200,000,000            
Senior Secured Revolving Credit Facility | Secured Debt                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity           $ 2,000,000,000.0    
Current borrowing capacity         $ 1,100,000,000      
Loss on extinguishment of debt $ 4,900,000              
Borrowings outstanding   200,000,000            
6.25% Senior Notes due 2023                
Line of Credit Facility [Line Items]                
Debt instrument, interest rate, stated (as a percent) 6.25%     6.25%       6.25%
Borrowings outstanding $ 650,000,000 0            
6.125% Senior Notes due 2024                
Line of Credit Facility [Line Items]                
Debt instrument, interest rate, stated (as a percent) 6.125%   6.125%       6.125% 6.125%
Borrowings outstanding $ 600,000,000 $ 600,000,000            
New Credit Facility                
Line of Credit Facility [Line Items]                
Maximum borrowing capacity       $ 5,000,000,000.0        
Current borrowing capacity 2,500,000,000              
Increase limit 2,000,000,000.0              
Borrowings outstanding $ 1,285,000,000              
Leverage ratio 300.00%              
Letters of credit outstanding $ 17,700,000              
Interest rate at period end (as a percent) 3.56%              
New Credit Facility | Minimum                
Line of Credit Facility [Line Items]                
Unused capacity, commitment fee (as a percent) 0.375%              
New Credit Facility | Maximum                
Line of Credit Facility [Line Items]                
Unused capacity, commitment fee (as a percent) 0.50%              
New Credit Facility | Base Rate | Minimum                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 0.25%              
New Credit Facility | Base Rate | Maximum                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 1.25%              
New Credit Facility | Federal Funds Rate                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 0.50%              
New Credit Facility | LIBOR                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 1.00%              
New Credit Facility | Eurodollar | Minimum                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 1.25%              
New Credit Facility | Eurodollar | Maximum                
Line of Credit Facility [Line Items]                
Debt instrument interest rate 2.25%