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Debt (Credit Facilities) (Narrative) (Details) - USD ($)
3 Months Ended 4 Months Ended 12 Months Ended
Jun. 30, 2018
Aug. 20, 2018
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]        
Commercial paper issued     $ 9,300,000,000  
Commercial paper repaid     9,300,000,000  
Commercial paper outstanding     $ 200,000,000 $ 200,000,000
Commercial paper maturity duration minimum     1 day  
Commercial paper maturity duration maximum     32 days  
Bilateral uncommitted line of credit [Member]        
Debt Instrument [Line Items]        
Facility expiration date   Aug. 20, 2018    
Line of credit facility available credit   $ 1,000,000,000.0    
Amount drawn and repaid under line of credit $ 750,000,000      
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Credit facility term     5 years  
Line of credit facility available credit     $ 2,000,000,000.0  
Revolving credit facility withdrawals     0  
Allowable debt per debt-to-net-worth coverage ratio (as defined in the facility)     38,500,000,000  
Outstanding debt (as defined by facility)     26,400,000,000  
Cross-default provision (as defined by facility)     $ 150,000,000