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Investments - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 6,869	$ 7,119
Fixed-maturity securities, trading	300	303
Short-term investments, at fair value	1,273	810
Other invested assets	140	133
Total investments	8,582	8,365
Externally managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	5,498	5,519
Loss mitigation securities and other
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	696	705
AssuredIM managed
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	539	537
Recovery Bonds
Debt and Equity Securities, FV-NI [Line Items]
Estimated fair value	$ 136	$ 358
BIG
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	7.50%	7.40%
Not Internally Rated
Debt and Equity Securities, FV-NI [Line Items]
Fixed-maturity investments, non-investment grade, percent	2.90%	5.90%
Equity method investments
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 122	$ 123
Other
Debt and Equity Securities, FV-NI [Line Items]
Other invested assets	$ 18	$ 10

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Fixed-Maturity Investments, Non-Investment Grade, Percent

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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