Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Amortized Cost	$ 10,296	$ 10,613
Estimated Fair Value
Estimated Fair Value	10,733	10,818
Fixed Maturities [Member]
Amortized Cost
Due within one year	223
Due after one year through five years	1,637
Due after five years through 10 years	2,101
Due after 10 years	3,784
Amortized Cost	9,137	9,884
Estimated Fair Value
Due within one year	216
Due after one year through five years	1,665
Due after five years through 10 years	2,196
Due after 10 years	4,064
Estimated Fair Value	9,574	10,089
Fixed Maturities [Member] | RMBS [Member]
Amortized Cost
Amortized Cost	914	999
Estimated Fair Value
Estimated Fair Value	938	982
Fixed Maturities [Member] | CMBS [Member]
Amortized Cost
Amortized Cost	478	542
Estimated Fair Value
Estimated Fair Value	$ 495	$ 539