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Debt - Five-Year Revolving Credit Facility (Details) - Unsecured Debt
12 Months Ended
Sep. 11, 2019
USD ($)
Dec. 31, 2019
USD ($)
subsidiary
Sep. 30, 2015
USD ($)
Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity $ 5,000,000,000.0    
Increase limit 2,000,000,000.0    
Number of subsidiaries designated as additional borrowers | subsidiary   0  
Borrowings outstanding   $ 0  
Remaining borrowing capacity   5,000,000,000.0  
Credit Agreement | Letter of Credit      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity 150,000,000    
Borrowings outstanding   $ 0  
Credit Agreement | Bridge Loan      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity $ 500,000,000    
Terminated Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Maximum borrowing capacity     $ 2,000,000,000.0
Credit facility, term 5 years    
Minimum | Eurodollar | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   0.875%  
Minimum | Overnight Rate | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   0.875%  
Minimum | Prime Rate, The Federal Funds Effective Rate Or London Interbank Offered Rate (LIBOR) | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   0.00%  
Maximum | Eurodollar | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   1.375%  
Maximum | Overnight Rate | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   1.375%  
Maximum | Prime Rate, The Federal Funds Effective Rate Or London Interbank Offered Rate (LIBOR) | Credit Agreement | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Basis spread on variable rate   0.375%