Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information
Total Assets	$ 64,140.5	$ 63,255.2
Operating Segments | Unum US
Segment Reporting Information
Total Assets	15,365.4	15,561.1
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,569.4	3,372.9
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,994.2	4,830.4
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	35,527.4	35,272.8
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 4,684.1	$ 4,218.0