Segment Information Segments - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2026 USD ($) Integer	Sep. 30, 2025 USD ($)	Dec. 31, 2025 USD ($)
Segment Reporting
NumberOfReportableSegmentsDisclosedByDefinitionFlag	true		true
Goodwill	$ 352.9		$ 352.9		$ 353.9
Number of Operating Segments | Integer			3
Strategic Actions	0.0	$ 0.0	$ 30.7	$ 0.0
Strategic Actions Real Estate Related Costs			18.0
Strategic Actions included within net investment income	0.0	$ 0.0	16.3	$ 0.0
Strategic Actions Employee-Related Costs			12.7
Strategic Actions included within labor and related expenses			11.2
Unum International
Segment Reporting
Goodwill	44.0		44.0		45.0
Unum US
Segment Reporting
Goodwill	281.2		281.2		281.2
Colonial Life
Segment Reporting
Goodwill	$ 27.7		$ 27.7		$ 27.7