Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information
Total Assets	$ 63,255.2	$ 61,148.5
Operating Segments | Unum US
Segment Reporting Information
Total Assets	15,561.1	16,356.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,372.9	3,129.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,830.4	4,575.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	35,272.8	33,776.6
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 4,218.0	$ 3,311.2