Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting Information
Total Assets	$ 69,256.6	$ 70,625.8
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,563.1	19,034.2
Operating Segments | Unum International
Segment Reporting Information
Total Assets	4,163.6	4,206.2
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,817.3	4,864.3
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	37,845.1	38,187.2
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 3,867.5	$ 4,333.9