Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Total Assets	$ 61,434.9	$ 70,115.6
Operating Segments | Unum US
Segment Reporting Information
Total Assets	16,278.7	18,696.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,154.9	4,086.5
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,448.9	4,895.9
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,241.2	38,287.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 3,311.2	$ 4,149.0