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Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		9 Months Ended
Reinsurance (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Sep. 30, 2025	Dec. 31, 2025	Jul. 01, 2025
Effects of Reinsurance
Debt Securities, Available-for-Sale	$ 34,443.6			$ 34,443.6			$ 33,056.6
Accrued Investment Income	681.1			681.1			596.0
Gain (Loss) on Investments	(5.2)		$ (17.7)	(10.2)	$ (224.5)
Reinsurance Recoverable	$ 11,091.3			$ 11,091.3			11,574.6
2025 Fortitude Re Reinsurance Transaction
Effects of Reinsurance
Cash Related to Reinsurance Agreement						$ 953.5
Negative Ceding Commission							$ 442.3
Debt Securities, Available-for-Sale								$ 3,230.1
Accrued Investment Income								47.1
Reinsurance Recoverable								3,620.5
2025 Fortitude Re Reinsurance Transaction \| Long-term Care \| Closed Block
Effects of Reinsurance
Reinsurance Recoverable								3,315.2
DeferredLossOnReinsurance								848.2
2025 Fortitude Re Reinsurance Transaction \| Individual Disability \| Unum US
Effects of Reinsurance
Reinsurance Recoverable								305.3
DeferredGainOnReinsurance								$ 145.9
Change in Deferred Acquisition Costs		$ 100.3