Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Total Assets	$ 70,625.8	$ 67,013.4
Operating Segments | Unum US
Segment Reporting Information
Total Assets	19,034.2	18,586.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	4,206.2	3,869.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,864.3	4,629.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	38,187.2	37,008.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 4,333.9	$ 2,920.3