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Reinsurance - Additional Information (Detail) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2025 USD ($) | Dec. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Jul. 01, 2025 USD ($)
Cash Related to Reinsurance Transaction |  |  | $ 945.3 | $ 0.0 | $ 0.0 |
Fixed Maturity Securities |  |  | 33,056.6 | 35,629.9 |  |
Accrued Investment Income |  |  | 596.0 | 649.8 |  |
Negative Ceding Commission |  |  | 442.3 |  |  |
Net Investment Loss |  |  | 106.6 | 34.6 | 36.0 |
Reinsurance Recoverables |  |  | 11,574.6 | 8,296.4 |  |
Amortization of Deferred Loss on Reinsurance |  |  | 116.7 | 41.4 | 44.1 |
Amortization of Deferred Gain on Reinsurance |  |  | $ 9.0 | 0.0 | 0.0 |
Number of companies that make up majority of reinsurance recoverable |  |  | 8 |  |  |
Operating Segments |  |  |  |  |  |
Net Investment Loss |  |  | $ 106.6 | 34.6 | 36.0 |
Amortization of Deferred Gain on Reinsurance |  |  | 9.0 | 0.0 | 0.0 |
Long-term Care and Individual Disability Reinsurance Transaction |  |  |  |  |  |
Cash Related to Reinsurance Transaction |  | $ 953.5 |  |  |  |
Fixed Maturity Securities |  |  |  |  |  | $ 3,230.1
Accrued Investment Income |  |  |  |  |  | $ 47.1
Reinsurance Recoverables |  |  | 3,620.5 |  |  |
Long-term Care and Individual Disability Reinsurance Transaction | Unum US |  |  |  |  |  |
Change in Deferred Acquisition Costs | [1] |  | 100.3 |  |  |
Long-term Care and Individual Disability Reinsurance Transaction | Individual Disability | Unum US |  |  |  |  |  |
Reinsurance Recoverables |  |  | 305.3 |  |  |
DeferredGainOnReinsurance |  |  | 145.9 |  |  |
Change in Deferred Acquisition Costs | [1] |  | 100.3 |  |  |
Amortization of Deferred Gain on Reinsurance |  |  | 9.0 |  |  |
Long-term Care and Individual Disability Reinsurance Transaction | Long-term Care | Closed Block |  |  |  |  |  |
Reinsurance Recoverables |  |  | 3,315.2 |  |  |
DeferredLossOnReinsurance |  |  | 848.2 |  |  |
Amortization of Deferred Loss on Reinsurance |  |  | 78.0 |  |  |
Long-term Care and Individual Disability Reinsurance Transaction | Operating Segments |  |  |  |  |  |
Net Investment Loss |  |  | 46.8 | 0.0 | 0.0 |
Change in Deferred Acquisition Costs | [1] |  | 100.3 |  |  |
Closed Block Individual Disability Reinsurance Transaction |  |  |  |  |  |
Reinsurance Recoverables |  |  | 4,561.3 | 4,698.4 |  |
Amortization of Deferred Loss on Reinsurance |  |  | 38.7 | 41.4 | $ 44.1 |
Deposit Contracts, Assets |  |  | $ 270.5 | $ 282.3 |  |
AM Best A Rating or Better | Reinsurer Concentration Risk | Reinsurance Recoverable | Top 9 Reinsurers |  |  |  |  |  |
Percentage of Concentration Risk |  |  | 25.00% |  |  |
AM Best A Rating or Better | Reinsurer Concentration Risk | Reinsurance Recoverable | Top 2 reinsurers |  |  |  |  |  |
Percentage of Concentration Risk |  |  | 72.00% |  |  |
AM Best, A- Rating | Reinsurer Concentration Risk | Reinsurance Recoverable | Other Reinsurers |  |  |  |  |  |
Percentage of Concentration Risk |  |  | 3.00% |  |  |
AM Best, Below A- or Not Rated | Reinsurer Concentration Risk | Reinsurance Recoverable | Other Reinsurers |  |  |  |  |  |
Percentage of Concentration Risk |  |  | 1.00% |  |  |
[1] | 1 Reflects the impacts of DAC written off related to the Fortitude Re reinsurance transaction. See Note 14 for further discussion of the reinsurance transaction.

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