Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Total Assets	$ 67,013.4	$ 61,875.6
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,586.3	17,510.9
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,869.1	3,426.8
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,629.0	4,237.9
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	37,008.7	34,527.6
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 2,920.3	$ 2,172.4