Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Total Assets	$ 65,335.9	$ 67,013.4
Operating Segments | Unum US
Segment Reporting Information
Total Assets	17,791.5	18,586.3
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,515.1	3,869.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,428.0	4,629.0
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	36,004.3	37,008.7
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 3,597.0	$ 2,920.3