Investments (Concentrations of Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investments in Japanese government and government agency securities:
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	$ 52,191	$ 53,997
Fair Value	59,182	61,866
Investments in Japanese government and government agency securities: | Fixed maturities, available-for-sale
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	50,735	52,703
Fair Value	57,562	60,379
Investments in Japanese government and government agency securities: | Fixed maturities, held-to-maturity
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	783	801
Fair Value	940	981
Investments in Japanese government and government agency securities: | Trading account assets supporting insurance liabilities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	475	457
Fair Value	482	470
Investments in Japanese government and government agency securities: | Other trading account assets
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	35	36
Fair Value	35	36
Investments in Japanese government and government agency securities: | Short-term investments
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	0	0
Fair Value	0	0
Investments in Japanese government and government agency securities: | Cash equivalents
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	163	0
Fair Value	163	0
Investments in South Korean government and government agency securities:
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	7,251	6,976
Fair Value	8,860	8,488
Investments in South Korean government and government agency securities: | Fixed maturities, available-for-sale
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	7,207	6,927
Fair Value	8,815	8,438
Investments in South Korean government and government agency securities: | Fixed maturities, held-to-maturity
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	0	0
Fair Value	0	0
Investments in South Korean government and government agency securities: | Trading account assets supporting insurance liabilities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	44	49
Fair Value	45	50
Investments in South Korean government and government agency securities: | Other trading account assets
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	0	0
Fair Value	0	0
Investments in South Korean government and government agency securities: | Short-term investments
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	0	0
Fair Value	0	0
Investments in South Korean government and government agency securities: | Cash equivalents
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Amortized Cost	0	0
Fair Value	$ 0	$ 0