Segment Information Segments - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($) Integer	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($)
Segment Reporting
Goodwill	$ 353.0		$ 353.0		$ 353.9
Number of Operating Segments | Integer			3
Strategic Actions	30.7	$ 0.0	$ 30.7	$ 0.0
Strategic Actions included within labor and related expenses			11.2
Strategic Actions included within net investment income	$ 16.3	$ 0.0	$ 16.3	$ 0.0
NumberOfReportableSegmentsDisclosedByDefinitionFlag	true		true
Unum International
Segment Reporting
Goodwill	$ 44.1		$ 44.1		45.0
Unum US
Segment Reporting
Goodwill	281.2		281.2		281.2
Colonial Life
Segment Reporting
Goodwill	$ 27.7		$ 27.7		$ 27.7