Investments and Cash - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	$ 9,414	$ 10,122
Short-term investments at fair value	858	1,268
Other invested assets	113	118
Total investment portfolio	9,527	10,240
Externally managed
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	7,237	7,978
AssuredIM
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	520	0
Loss mitigation and other securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity securities	799	876
Equity Method Investments, Internally Managed
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	97	111
Other Investments, Internally Managed
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 16	$ 7
BIG
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	8.10%	8.60%