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Investments (Concentrations of Credit Risk) (Details) - Investments in Japanese government and government agency securities: - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Concentrations of credit risk at amortized cost $ 76,845 $ 82,059
Concentrations of credit risk at fair value 87,747 94,817
Assets supporting experience-rated contractholder liabilities    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Concentrations of credit risk at amortized cost 830 849
Concentrations of credit risk at fair value 829 855
Available-for-sale | Fixed maturities    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Concentrations of credit risk at amortized cost 75,142 80,273
Concentrations of credit risk at fair value 85,813 92,764
Held-to-maturity | Fixed maturities    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Concentrations of credit risk at amortized cost 850 912
Concentrations of credit risk at fair value 1,082 1,173
Trading | Fixed maturities    
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]    
Concentrations of credit risk at amortized cost 23 25
Concentrations of credit risk at fair value $ 23 $ 25