Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Total Assets	$ 61,875.6	$ 64,013.1
Operating Segments
Segment Reporting Information
Total Assets	61,875.6	64,013.1
Operating Segments | Unum US
Segment Reporting Information
Total Assets	17,510.9	18,109.1
Operating Segments | Unum International
Segment Reporting Information
Total Assets	3,426.8	3,428.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,237.9	4,184.1
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	34,527.6	35,051.2
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 2,172.4	$ 3,240.6