Segment Information Segments - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) Integer	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Supplementary Insurance Information, by Segment
Number of Operating Segments | Integer	3
Goodwill	$ 349.9	$ 347.6
Operating Segments
Supplementary Insurance Information, by Segment
Capitalized Computer Software, Impairments	0.0	0.0	$ 12.1
Unum US
Supplementary Insurance Information, by Segment
Goodwill	280.0	280.0
Unum International
Supplementary Insurance Information, by Segment
Goodwill	42.2	39.9
Colonial Life
Supplementary Insurance Information, by Segment
Goodwill	$ 27.7	$ 27.7