Short-Term and Long-Term Debt (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Non-operating indebtedness of subsidiaries to total capitalization, maximum	7.50%
Debt instrument, Consecutive years of non-payment of interest resulting in default	10 years
Capital Securities [Member]
Debt Instrument [Line Items]
Principal balance	$ 1,200,000,000
Risk based capital ratio threshold	175.00%
Capital securities covenant percentage	10.00%
Five-year revolving credit facility [Member]
Debt Instrument [Line Items]
Maximum Issuance Of Line of Credit	$ 2,250,000,000
Minimum consolidated net worth	$ 10,600,000,000
Percentage of aggregate net proceeds of equity issuances	50.00%
Debt to capital ratio (low end of range)	0.35%
Debt to capital ratio (high end of range)	1.00%
Delaware Trust Facility [Member]
Debt Instrument [Line Items]
Facility agreement term	10 years
Pre-capitalized securities, sold by trust	$ 500,000,000
Facility fee rate	1.691%
Stockholders' equity threshold triggering required facility exercise	$ 2,750,000,000
Potential Issuance In Exchange For US Treasury Securities [Member] | Senior Notes 2.330%, Due 2030 [Member]
Debt Instrument [Line Items]
Interest rate	2.33%
Potential Issuance In Exchange For US Treasury Securities [Member] | Delaware Trust Facility [Member] | Senior Notes 2.330%, Due 2030 [Member]
Debt Instrument [Line Items]
Principal balance	$ 500,000,000
Interest rate	2.33%