Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	$ 9,451	$ 10,122
Short-term investments at fair value	821	1,268
Other invested assets (includes $15 and $6 measured at fair value)	122	118
Total investment portfolio	9,573	10,240
Securities, Managed Externally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	7,503	7,978
Securities, Managed Internally [Member]
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	773	876
Securities, Managed Internally by Assured Investment Management [Member]
Debt and Equity Securities, FV-NI [Line Items]
Unrealized loss position	354	0
Other Investments, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets (includes $15 and $6 measured at fair value)	15	7
Equity Method Investments, Internally Managed [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets (includes $15 and $6 measured at fair value)	$ 107	$ 111
BIG [Member]
Debt and Equity Securities, FV-NI [Line Items]
Fixed-Maturity Investments, Non-Investment Grade, Percent	7.70%	8.60%