Investments - Fixed Maturities by Contractual Maturity Periods (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost:
Due in one year or less	$ 1,561
Due after one year through five years	6,678
Due after five years through ten years	10,430
Due after ten years	3,935
Mortgage and other asset-backed securities	519
Amortized Cost	23,123	$ 21,867
Fair Value:
Due in one year or less	1,569
Due after one year through five years	6,760
Due after five years through ten years	10,242
Due after ten years	4,494
Mortgage and other asset-backed securities	528
Total fair value	$ 23,593