Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Cost-method Investments [Line Items]
Fixed-maturity securities	$ 11,048	$ 11,301
Other invested assets	103	94
Total investment portfolio	11,151	11,395
Fixed Maturities, Managed Internally [Member]
Schedule of Cost-method Investments [Line Items]
Fixed-maturity securities	1,199	1,231
Other Invested Assets, Internally Managed [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	18	20
Alternative Investments [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	80	69
Other, Internally Managed [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	5	5
Internally Managed Portfolio [Member]
Schedule of Cost-method Investments [Line Items]
Total investment portfolio	$ 1,302	$ 1,325