Investments (Concentrations of Credit Risk) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentration of credit risk at amortized cost	$ 82,059	$ 75,663
Concentration of credit risk at fair value	94,817	91,376
Investments in South Korean government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentration of credit risk at amortized cost	41	10,838
Concentration of credit risk at fair value	49	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]
Concentration of credit risk at amortized cost	849	653
Concentration of credit risk at fair value	855	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]
Concentration of credit risk at amortized cost	15	15
Concentration 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]
Concentration of credit risk at amortized cost	80,273	74,118
Concentration of credit risk at fair value	92,764	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]
Concentration of credit risk at amortized cost	26	10,823
Concentration of credit risk at fair value	33	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]
Concentration of credit risk at amortized cost	912	869
Concentration of credit risk at fair value	1,173	1,143
Trading | Fixed Maturities | Investments in Japanese government and government agency securities:
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Concentration of credit risk at amortized cost	25	23
Concentration of credit risk at fair value	$ 25	$ 23