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Medium- And Long-Term Debt (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Unamortized debt issuance costs $ 5 $ 7
Subsidiaries    
Debt Instrument [Line Items]    
Blanket lien on real-estate related loans securing FHLB advances 16,000  
FHLB potential future borrowings 5,000  
Floating-rate FHLB advances | Subsidiaries    
Debt Instrument [Line Items]    
Floating-rate FHLB advances $ 2,800 $ 2,800
Weighted-average interest rate 1.41%  
Minimum | Floating-rate FHLB advances | Subsidiaries    
Debt Instrument [Line Items]    
Maturity date Mar. 04, 2026  
Maximum | Floating-rate FHLB advances | Subsidiaries    
Debt Instrument [Line Items]    
Maturity date May 20, 2026