Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Segment Reporting
Total Assets	$ 62,714.3	$ 63,519.4
Operating Segments | Unum US
Segment Reporting
Total Assets	14,500.9	14,635.3
Operating Segments | Unum International
Segment Reporting
Total Assets	3,661.9	3,648.0
Operating Segments | Colonial Life
Segment Reporting
Total Assets	5,332.4	5,289.9
Operating Segments | Closed Block
Segment Reporting
Total Assets	32,503.2	33,887.8
Operating Segments | Corporate
Segment Reporting
Total Assets	$ 6,715.9	$ 6,058.4