Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information
Total Assets	$ 70,115.6	$ 70,625.8
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,696.3	19,034.2
Operating Segments | Unum International
Segment Reporting Information
Total Assets	4,086.5	4,206.2
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,895.9	4,864.3
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	38,287.9	38,187.2
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 4,149.0	$ 4,333.9