Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Sep. 30, 2025	Dec. 31, 2025	Jul. 01, 2025
Effects of Reinsurance
Debt Securities, Available-for-Sale	$ 33,300.8			$ 33,056.6
Accrued Investment Income	582.0			596.0
Net Investment Loss	(5.0)	$ (206.8)
Reinsurance Recoverable	11,310.9			11,574.6
Operating Segments
Effects of Reinsurance
Net Investment Loss	(5.0)	(206.8)
Long-term Care and Individual Disability 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
Reinsurance Recoverable				3,620.5
Long-term Care and Individual Disability Reinsurance Transaction | Operating Segments
Effects of Reinsurance
Net Investment Loss	$ 0.0	$ (175.9)		(46.8)
Long-term Care and Individual Disability Reinsurance Transaction | Long-term Care | Closed Block
Effects of Reinsurance
Reinsurance Recoverable				3,315.2
Deferred Loss On Reinsurance				848.2
Long-term Care and Individual Disability Reinsurance Transaction | Individual Disability | Unum US
Effects of Reinsurance
Reinsurance Recoverable				305.3
Deferred Gain On Reinsurance				145.9
Change in Deferred Acquisition Costs				$ (100.3)