Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Segment Reporting Information
Total Assets	$ 62,044.4	$ 63,255.2
Operating Segments | Unum US
Segment Reporting Information
Total Assets	15,100.2	15,561.1
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,375.8	3,372.9
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,856.1	4,830.4
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,308.5	35,272.8
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 4,403.8	$ 4,218.0