Investments - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 9,357	$ 9,624
Short-term investments, at fair value	694	851
Other invested assets	260	214
Total investments	$ 9,617	$ 9,838
BIG
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	7.90%	8.10%
Externally managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 7,214	$ 7,301
Loss mitigation securities and other
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	900	925
AssuredIM managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	549	547
Equity method investments - AssuredIM Funds
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	102	91
Equity method investments - other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	145	107
Other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 13	$ 16