Investments (Concentrations of Credit Risk) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	$ 76,131	$ 75,663
Concentrations of credit risk at fair value	91,186	91,376
Investments in South Korean government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	10,503	10,838
Concentrations of credit risk at fair value	12,942	13,338
Assets supporting experience-rated contractholder liabilities | Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	652	653
Concentrations of credit risk at fair value	659	664
Assets supporting experience-rated contractholder liabilities | Investments in South Korean government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	15	15
Concentrations of credit risk at fair value	16	16
Available-for-sale | Fixed maturities | Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	74,584	74,118
Concentrations of credit risk at fair value	89,377	89,546
Available-for-sale | Fixed maturities | Investments in South Korean government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	10,488	10,823
Concentrations of credit risk at fair value	12,926	13,322
Held-to-maturity | Fixed maturities | Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	873	869
Concentrations of credit risk at fair value	1,128	1,143
Trading | Fixed maturities | Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentrations of credit risk at amortized cost	22	23
Concentrations of credit risk at fair value	$ 22	$ 23