Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Segment Reporting Information
Total Assets	$ 60,866.7	$ 70,115.6
Operating Segments | Unum US
Segment Reporting Information
Total Assets	16,183.0	18,696.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	2,910.5	4,086.5
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,412.5	4,895.9
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,876.8	38,287.9
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 3,483.9	$ 4,149.0