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Investments - Additional Information (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 31, 2025 USD ($) | Sep. 30, 2025 USD ($) security | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) security | Sep. 30, 2024 USD ($) | Jun. 30, 2025 USD ($) | Dec. 31, 2024 USD ($) security
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Fixed-maturity investments, non-investment grade, percent |  | 8.80% |  | 8.80% |  |  | 12.60%
Estimated Fair Value |  | $ 6,278 |  | $ 6,278 |  |  | $ 6,369
Alternative investment |  | 945 |  | 945 |  |  |
Accrued investment income |  | $ 70 |  | $ 70 |  |  | $ 64
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | security |  | 277 |  | 277 |  |  | 438
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more |  | $ 151 |  | $ 151 |  |  | $ 223
Assets held-in-trust |  | 82 |  | 82 |  |  | 79
Fixed-maturity securities, trading |  | 136 |  | 136 |  |  | 147
Proceeds from sale and maturity of debt securities, available-for-sale |  |  |  | 533 | $ 575 |  |
Securities purchased with credit deterioration |  | 0 | $ 0 | 0 | $ 0 |  |
Insured Financial Obligations, Loss Mitigation Securities |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Estimated Fair Value |  | 140 |  | 140 |  |  | 479
Insured Financial Obligations, Loss Mitigation Securities Paid Down |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Estimated Fair Value |  |  |  |  |  | $ 408 |
Investment resolution, amount received | $ 459 |  |  |  |  |  |
Collateralized Loan Obligation Equity Tranches |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Estimated Fair Value |  | 242 |  | 242 |  |  | 277
Consolidated Investment Vehicles |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Equity method investments |  | 45 |  | 45 |  |  | 33
Contingent Value Instruments |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Fixed-maturity securities, trading |  | 125 |  | 125 |  |  |
Financial Instrument, Other |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Investments, fair value disclosure |  | 598 |  | 598 |  |  |
Sound Point Capital Management, LP |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Equity method investments |  | 414 |  | 414 |  |  | 418
AGL Subsidiaries |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Assets held-in-trust |  | 789 |  | 789 |  |  | $ 1,135
Future Equity Investments |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Alternative investment |  | 945 |  | 945 |  |  |
Investments, unfunded commitment |  | 563 |  | 563 |  |  |
Investments, authorized amount to invest |  | 1,500 |  | 1,500 |  |  |
Future Equity Investments | Sound Point Capital Management, LP |  |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |  |
Investments, authorized amount to invest |  | $ 1,000 |  | $ 1,000 |  |  |

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