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Short-Term and Long-Term Debt (Narrative) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
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 $ 2,500,000,000
Current borrowing capacity 2,500,000,000
Borrowing capacity available to reimburse the banks for drawn LOCs 1,750,000,000
Minimum consolidated net worth $ 10,500,000,000
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%