Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Amortized Cost	$ 10,720	$ 10,814
Estimated Fair Value
Estimated Fair Value	11,048	11,301
Fixed Maturities [Member]
Amortized Cost
Due within one year	274
Due after one year through five years	1,438
Due after five years through 10 years	2,345
Due after 10 years	4,494
Amortized Cost	9,969	10,187
Estimated Fair Value
Due within one year	272
Due after one year through five years	1,467
Due after five years through 10 years	2,378
Due after 10 years	4,772
Estimated Fair Value	10,297	10,674
Fixed Maturities [Member] | RMBS [Member]
Amortized Cost
Amortized Cost	857	852
Estimated Fair Value
Estimated Fair Value	849	861
Fixed Maturities [Member] | CMBS [Member]
Amortized Cost
Amortized Cost	561	540
Estimated Fair Value
Estimated Fair Value	$ 559	$ 549