Investments (Concentrations of Credit Risk) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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	$ 75,663	$ 73,529
Concentration of credit risk at fair value	91,376	86,307
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	10,838	10,354
Concentration of credit risk at fair value	13,338	12,601
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	653	691
Concentration of credit risk at fair value	664	697
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	15
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	74,118	71,952
Concentration of credit risk at fair value	89,546	84,461
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	10,823	10,339
Concentration of credit risk at fair value	13,322	12,586
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	869	864
Concentration of credit risk at fair value	1,143	1,127
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	23	22
Concentration of credit risk at fair value	$ 23	$ 22