Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	$ 10,930	$ 11,301
Other invested assets	95	94
Total investment portfolio	11,025	11,395
Fixed Maturities, Managed Internally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	1,184	1,231
Other Invested Assets, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	13	20
Gain on sale of equity security	31
Alternative Investments [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	78	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	$ 1,279	$ 1,325