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Debt (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Debt Instrument [Line Items]        
Interest and debt expense $ 73 $ 66 $ 204 $ 218
Capital Securities [Member]        
Debt Instrument [Line Items]        
Interest and debt expense 8      
Line of Credit [Member]        
Debt Instrument [Line Items]        
Current borrowing capacity 2,500   2,500  
Minimum consolidated net worth $ 10,000   $ 10,000  
Percentage of aggregate net proceeds of equity issuances 50.00%   50.00%  
Debt to capital ratio (low end of range) 0.35%   0.35%  
Debt to capital ratio (high end of range) 1.00%   1.00%  
Non-operating indebtedness of subsidiaries to total capitalization, maximum     7.50%