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Segment Information Segments - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) Integer	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Supplementary Insurance Information, by Segment
Number of Operating Segments \| Integer	3
Goodwill	$ 353.9	$ 349.1
Loss on Legal Settlement		15.3
Number of Reportable Segments \| Integer	5
Compensation Expense
Supplementary Insurance Information, by Segment
Loss on Legal Settlement	$ 4.9
Other Expense
Supplementary Insurance Information, by Segment
Loss on Legal Settlement	10.4
Operating Segments
Supplementary Insurance Information, by Segment
Loss on Legal Settlement	0.0	15.3	$ 0.0
Unum US
Supplementary Insurance Information, by Segment
Goodwill	281.2	280.0
Unum International
Supplementary Insurance Information, by Segment
Goodwill	45.0	41.4
Colonial Life
Supplementary Insurance Information, by Segment
Goodwill	$ 27.7	$ 27.7