Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information
Total Assets	$ 61,959.3	$ 63,255.2
Operating Segments
Segment Reporting Information
Total Assets	61,959.3	63,255.2
Operating Segments | Unum US
Segment Reporting Information
Total Assets	14,981.6	15,561.1
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,291.3	3,372.9
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,964.2	4,830.4
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,376.0	35,272.8
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 5,346.2	$ 4,218.0