Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Total Assets	$ 64,013.1	$ 61,941.5
Operating Segments
Segment Reporting Information
Total Assets	64,013.1	61,941.5
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,109.1	18,036.6
Operating Segments | Unum UK
Segment Reporting Information
Total Assets	3,428.1	3,101.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	4,184.1	3,923.2
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	35,051.2	33,734.3
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 3,240.6	$ 3,146.0