Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	$ 9,501	$ 10,122
Short-term investments at fair value	933	1,268
Other invested assets (includes $12 and $6 measured at fair value)	121	118
Total investment portfolio	9,622	10,240
Securities, Managed Externally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	7,826	7,978
Other Investments, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets (includes $12 and $6 measured at fair value)	13	7
Securities, Managed Internally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	742	876
Equity Method Investments, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets (includes $12 and $6 measured at fair value)	$ 108	$ 111
BIG [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-Maturity Investments, Non-Investment Grade, Percent	7.60%	8.60%