Investments and Cash - Distribution of Fixed-Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Amortized Cost	$ 10,993	$ 10,564
Estimated Fair Value
Estimated Fair Value	11,495	10,823
Fixed Maturities [Member]
Amortized Cost
Due within one year	249
Due after one year through five years	1,518
Due after five years through 10 years	2,280
Due after 10 years	4,581
Amortized Cost	10,045	9,974
Estimated Fair Value
Due within one year	250
Due after one year through five years	1,552
Due after five years through 10 years	2,365
Due after 10 years	4,936
Estimated Fair Value	10,546	10,233
Fixed Maturities [Member] | RMBS [Member]
Amortized Cost
Amortized Cost	866	998
Estimated Fair Value
Estimated Fair Value	881	987
Fixed Maturities [Member] | CMBS [Member]
Amortized Cost
Amortized Cost	551	575
Estimated Fair Value
Estimated Fair Value	$ 562	$ 583