Financial Guaranty Variable Interest Entities and Consolidated Investment Vehicles - Additional Information (Details)	Mar. 31, 2025 USD ($) Entity policy	Dec. 31, 2024 USD ($) Entity policy	Mar. 31, 2024 USD ($)
Variable Interest Entity [Line Items]
Assets	$ 11,938,000,000	$ 11,901,000,000
Cash	$ 177,000,000	$ 121,000,000	$ 115,000,000
Number of policies monitored | policy	15,000
Number of policies monitored, not within the scope of ASC 810 | policy	14,000
Number of policies that contain provisions for consolidation | policy	50	50
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt	$ 766,000,000
Financial Guaranty Variable Interest Entities | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Maximum loss exposure	$ 0
Financial Guaranty Variable Interest Entities | Variable Interest Entity, Primary Beneficiary | Structured finance
Variable Interest Entity [Line Items]
Number of entities | Entity	23	23
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Total number of entities consolidated | Entity	1	1
Assets	$ 119,000,000	$ 101,000,000
Cash	1,000,000	2,000,000
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Affiliated Entity
Variable Interest Entity [Line Items]
Assets	$ 118,000,000	$ 99,000,000