Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Total Assets	$ 61,941.5	$ 60,563.6
Operating Segments
Segment Reporting Information
Total Assets	61,941.5	60,563.6
Operating Segments | Unum US
Segment Reporting Information
Total Assets	18,036.6	18,242.4
Operating Segments | Unum UK
Segment Reporting Information
Total Assets	3,101.4	3,432.4
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	3,923.2	3,776.8
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,734.3	33,000.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 3,146.0	$ 2,111.1