Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Amortized Cost	$ 10,693	$ 10,564
Estimated Fair Value
Estimated Fair Value	11,183	10,823
Fixed Maturities [Member]
Amortized Cost
Due within one year	250
Due after one year through five years	1,708
Due after five years through 10 years	2,262
Due after 10 years	4,363
Amortized Cost	10,018	9,974
Estimated Fair Value
Due within one year	250
Due after one year through five years	1,740
Due after five years through 10 years	2,338
Due after 10 years	4,716
Estimated Fair Value	10,505	10,233
Fixed Maturities [Member] | RMBS [Member]
Amortized Cost
Amortized Cost	878	998
Estimated Fair Value
Estimated Fair Value	894	987
Fixed Maturities [Member] | CMBS [Member]
Amortized Cost
Amortized Cost	557	575
Estimated Fair Value
Estimated Fair Value	$ 567	$ 583