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Investments (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]    
Interest-Bearing Deposits, Securities Purchased Under Agreements to Resell, and Federal Funds Sold, Percentage Rated Below Triple-B 0.00%  
Interest-Bearing Deposits, Securities Purchased Under Agreements to Resell, and Federal Funds Sold, Percentage Unrated 42.00%  
Securities Purchased under Agreements to Resell, Allowance for Credit Loss $ 0 $ 0
Available-for-Sale Debt Securities and Held-to-Maturity Debt Securities Excluding Private Label Mortgage Back Securities Amortized Cost, Percentage Rated Single-A Or Above 100.00%  
Debt Securities, Available-for-sale, Allowance for Credit Loss $ 0  
Debt Securities, Held-to-maturity, Allowance for Credit Loss $ 0  
Available-for-Sale Debt Securities and Held-to-Maturity Debt Securities Private Label Mortgage Back Securities Amortized Cost, Percentage Rated Single-A Or Above 70.00%  
Securities Borrowed or Purchased under Agreements to Resell [Member]    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Financial Asset, Amortized Cost, Accrued Interest, after Allowance for Credit Loss $ 0 2
Interest-Bearing Deposits and Federal Funds Sold [Member]    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Financing Receivable, Allowance for Credit Loss 0 0
Financial Asset, Amortized Cost, Accrued Interest, after Allowance for Credit Loss 0 $ 0
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Debt Securities, Held-to-maturity, Allowance for Credit Loss $ 0