Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Cost-method Investments [Line Items]
Fixed-maturity securities	$ 11,301	$ 10,823
Other invested assets	94	162
Total investment portfolio	11,395	10,985
Fixed Maturities, Managed Internally [Member]
Schedule of Cost-method Investments [Line Items]
Fixed-maturity securities	1,231	1,492
Other Invested Assets, Internally Managed [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	20	107
Alternative Investments [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	69	48
Other [Member]
Schedule of Cost-method Investments [Line Items]
Other invested assets	5	7
Internally Managed Portfolio [Member]
Schedule of Cost-method Investments [Line Items]
Total investment portfolio	$ 1,325	$ 1,654