Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Segment Reporting
Total Assets	$ 61,307.0	$ 63,519.4
Operating Segments | Unum US
Segment Reporting
Total Assets	14,357.1	14,635.3
Operating Segments | Unum International
Segment Reporting
Total Assets	3,673.2	3,648.0
Operating Segments | Colonial Life
Segment Reporting
Total Assets	5,478.1	5,289.9
Operating Segments | Closed Block
Segment Reporting
Total Assets	31,367.0	33,887.8
Operating Segments | Corporate
Segment Reporting
Total Assets	$ 6,431.6	$ 6,058.4