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Investments (Concentrations of Credit Risk - Narrative) (Details) - USD ($)
$ in Billions
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Securities holdings exposure in single issuer greater than stated percentage of Company's equity 10.00% 10.00%
Foreign government | Japan    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Government and agency fixed maturity securities $ 29.4 $ 20.9
Fixed maturity and equity securities | Japan    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Government and agency fixed maturity securities $ 33.8 $ 25.4