Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Segment Reporting Information
Total Assets	$ 59,954.5	$ 61,148.5	$ 60,377.9
Operating Segments | Unum US
Segment Reporting Information
Total Assets	15,088.5	16,356.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,165.9	3,129.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,608.6	4,575.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,070.4	33,776.6
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 4,021.1	$ 3,311.2