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Outstanding Exposure - Narrative (Details) - USD ($) $ in Millions | Jun. 26, 2023 | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | --- | ---
Schedule of Insured Financial Obligations [Line Items] |  |  |
Loss mitigation securities |  | $ 1,200.0 | $ 1,200.0
Net Par Outstanding |  | 261,552.0 | 249,153.0
Fixed-maturity securities, trading, at fair value |  | 147.0 | 318.0
Contingent Value Instruments |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Fixed-maturity securities, trading, at fair value |  | 123.0 |
PREPA | Guarantee Obligations |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Damages sought, estimate | $ 2,400.0 |  |
Damages sought, value | $ 8,500.0 |  |
Puerto Rico |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 637.0 | 1,105.0
Puerto Rico | Other Puerto Rico Exposures |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 92.0 | 109.0
Public finance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 250,375.0 | 239,296.0
Public finance | Commitment to Provide Guarantees |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Outstanding commitments to provide guaranties |  | 196.0 |
Potential future commitment |  |  | 353.0
Structured finance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 11,177.0 | 9,857.0
Structured finance | Commitment to Provide Guarantees |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Outstanding commitments to provide guaranties |  | 1,500.0 |
Potential future commitment |  |  | 50.0
Aircraft residual value insurance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Specialty business, gross par |  | 147.0 | 355.0
Specialty business, net par |  | 87.0 | 200.0
Specialty business, contingent, gross |  | 90.0 |
Specialty business, contingent, net |  | 51.0 |
BIG |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 10,182.0 | 5,437.0
BIG | Public finance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 9,286.0 | 4,402.0
BIG | Structured finance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Net Par Outstanding |  | 896.0 | 1,035.0
BIG | Aircraft residual value insurance |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Specialty business, gross par |  | 5.0 | 144.0
Specialty business, net par |  | $ 5.0 | $ 84.0
Minimum |  |  |
Schedule of Insured Financial Obligations [Line Items] |  |  |
Probability of paying more claims than being reimbursed (as a percent) |  | 50.00% |

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