Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information
Total Assets	$ 70,597.9	$ 70,625.8
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,898.8	19,034.2
Operating Segments | Unum International
Segment Reporting Information
Total Assets	4,202.7	4,206.2
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,918.1	4,864.3
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	38,227.1	38,187.2
Operating Segments | Corporate
Segment Reporting Information
Total Assets	$ 4,351.2	$ 4,333.9