Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Segment Information Segments - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) Integer	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Supplementary Insurance Information, by Segment
Number of Operating Segments \| Integer	3
Goodwill	$ 349.1	$ 349.9
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	15.3	0.0	$ 0.0
Unum US
Supplementary Insurance Information, by Segment
Goodwill	280.0	280.0
Unum International
Supplementary Insurance Information, by Segment
Goodwill	41.4	42.2
Colonial Life
Supplementary Insurance Information, by Segment
Goodwill	$ 27.7	$ 27.7