Investments (Concentrations of Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
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	$ 66,152	$ 61,611
Concentration of credit risk at fair value	78,104	74,692
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	9,440	7,625
Concentration of credit risk at fair value	11,004	9,479
Trading account assets | 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	657	537
Concentration of credit risk at fair value	667	550
Trading account assets | 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	44
Concentration of credit risk at fair value	15	44
Other trading account assets | 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	16
Concentration of credit risk at fair value	23	16
Other trading account assets | 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	0	0
Concentration of credit risk at fair value	0	0
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	64,628	60,240
Concentration of credit risk at fair value	76,311	73,051
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	9,425	7,581
Concentration of credit risk at fair value	10,989	9,435
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	844	818
Concentration of credit risk at fair value	1,103	1,075
Held-to-maturity | 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	0	0
Concentration of credit risk at fair value	$ 0	$ 0