Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Segment Reporting Information
Total Assets	$ 62,423.8	$ 61,148.5	$ 67,765.8
Operating Segments | Unum US
Segment Reporting Information
Total Assets	16,085.8	16,356.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,237.7	3,129.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,678.8	4,575.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,472.5	33,776.6
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 3,949.0	$ 3,311.2