Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information
Total Assets	$ 69,616.0	$ 67,013.4
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,910.0	18,586.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,974.7	3,869.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,771.7	4,629.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	38,158.3	37,008.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 3,801.3	$ 2,920.3