Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	$ 11,136	$ 11,301
Other invested assets	102	94
Total investment portfolio	11,238	11,395
Fixed Maturities, Managed Internally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	1,189	1,231
Other Invested Assets, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	17	20
Alternative Investments [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	81	69
Other, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	4	5
Internally Managed Portfolio [Member]
Debt and Equity Securities, FV-NI [Line Items]
Total investment portfolio	$ 1,291	$ 1,325