Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	$ 10,716	$ 10,818
Short-term investments, at fair value	727	729
Other invested assets	61	55
Total investment portfolio	10,777	10,873
Fixed Maturities, Managed Externally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	8,661	8,909
Fixed Maturities, Managed Internally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity securities	1,328	1,180
Alternative Investments [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	45	39
Other Investments, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 16	$ 16
BIG [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-Maturity Investments, Non-Investment Grade, Percent	10.90%	10.80%