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Indebtedness (Narrative) (Details) - USD ($)
Jan. 31, 2020
Sep. 30, 2020
Feb. 10, 2020
Feb. 09, 2020
Dec. 31, 2019
Line of Credit Facility [Line Items]          
Short-term borrowings, carrying value   $ 152,000,000     $ 338,000,000
Committed credit facilities   7,500,000,000      
Borrowings under committed credit facilities   0      
364-day revolving credit expiring February 2, 2021 [Member]          
Line of Credit Facility [Line Items]          
Committed credit facilities $ 2,000,000,000.0 2,000,000,000.0      
Debt instrument, term 364 days        
Multi-year Revolving Credit Facility, Expiring February 10, 2025 [Member]          
Line of Credit Facility [Line Items]          
Committed credit facilities   $ 2,000,000,000.0 $ 2,000,000,000.0    
Multi-year Revolving Credit Facility, Expiring February 10, 2020 [Member]          
Line of Credit Facility [Line Items]          
Committed credit facilities       $ 2,500,000,000  
Borrowings under committed credit facilities       $ 0