Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Total Assets	$ 66,471.7	$ 70,115.6
Operating Segments | Unum US
Segment Reporting Information
Total Assets	17,741.1	18,696.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,872.4	4,086.5
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,759.5	4,895.9
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	36,213.4	38,287.9
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 3,885.3	$ 4,149.0