Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Segment Reporting Information
Total Assets	$ 63,678.1	$ 61,959.3
Operating Segments | Unum US
Segment Reporting Information
Total Assets	14,792.9	14,981.6
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,612.3	3,291.3
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	5,190.9	4,964.2
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,080.0	33,376.0
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 6,002.0	$ 5,346.2