Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025	Nov. 03, 2026	Jul. 01, 2025
Effects of Reinsurance
Cash Related to Reinsurance Agreement					$ 0.0	$ 953.5
Debt Securities, Available-for-Sale	$ 32,994.5		$ 33,056.6		32,994.5		$ 33,056.6
Accrued Investment Income	751.3		596.0		751.3		596.0
Gain (Loss) on Investments	(853.0)			$ 128.0	(863.2)	(96.5)
Reinsurance Recoverable	10,525.8		11,574.6		10,525.8		11,574.6
Operating Segments
Effects of Reinsurance
Gain (Loss) on Investments	(853.0)			128.0	(863.2)	(96.5)
2025 Fortitude Re Reinsurance Transaction
Effects of Reinsurance
Cash Related to Reinsurance Agreement							953.5
Debt Securities, Available-for-Sale									$ 3,230.1
Accrued Investment Income									47.1
Negative Ceding Commission			442.3				$ 442.3
Reinsurance Recoverable									3,620.5
2025 Fortitude Re Reinsurance Transaction | Operating Segments
Effects of Reinsurance
Gain (Loss) on Investments		$ (46.8)		$ 137.6		(46.8)
Change in Deferred Acquisition Costs						100.3
2025 Fortitude Re Reinsurance Transaction | Unum US
Effects of Reinsurance
Change in Deferred Acquisition Costs						100.3
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			$ 100.3
2026 Fortitude Re Reinsurance Transaction | Subsequent Event
Effects of Reinsurance
Cash and Fixed Maturity Securities Transferred in Relation to Reinsurance Agreement								$ 5,500.7
2026 Fortitude Re Reinsurance Transaction | Operating Segments
Effects of Reinsurance
Gain (Loss) on Investments	$ (833.5)				$ (833.5)