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] 1Reflects the impacts of DAC written off related to the Fortitude Re reinsurance transaction. See Note 14 for further discussion of the reinsurance transaction.