Contracts Accounted for as Financial Guaranty Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Dec. 31, 2025 |
Sep. 30, 2025 |
Dec. 31, 2024 |
|
| Financial Guarantee Insurance Premiums [Line Items] | ||||
| Premiums receivable, net of commissions payable | $ 1,542 | $ 1,572 | $ 1,583 | $ 1,551 |
| Financial Guarantee Policies Paid in Installments | ||||
| Financial Guarantee Insurance Premiums [Line Items] | ||||
| Premiums receivable, net of commissions payable | 1,540 | 1,570 | ||
| Deferred premium revenue | $ 1,815 | $ 1,831 | ||
| Weighted-average risk-free rate used to discount premiums | 3.00% | 2.70% | ||
| Weighted-average period of premiums receivable (in years) | 12 years 1 month 6 days | 11 years 8 months 12 days |