Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information
Total Assets	$ 63,519.4	$ 61,959.3
Operating Segments
Segment Reporting Information
Total Assets	63,519.4	61,959.3
Operating Segments | Unum US
Segment Reporting Information
Total Assets	14,635.3	14,981.6
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,648.0	3,291.3
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	5,289.9	4,964.2
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,887.8	33,376.0
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 6,058.4	$ 5,346.2