Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Segment Reporting Information
Total Assets	$ 69,305.2	$ 67,013.4
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,790.7	18,586.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,823.9	3,869.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,768.0	4,629.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	37,823.7	37,008.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 4,098.9	$ 2,920.3