Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022
Segment Reporting Information
Total Assets	$ 61,904.4	$ 61,148.5	$ 63,510.1
Operating Segments | Unum US
Segment Reporting Information
Total Assets	15,813.2	16,356.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,315.2	3,129.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,674.9	4,575.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,309.8	33,776.6
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 3,791.3	$ 3,311.2