Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information
Assets	$ 63,805.7	$ 61,941.5
Operating Segments
Segment Reporting Information
Assets	63,805.7	61,941.5
Operating Segments | Unum US
Segment Reporting Information
Assets	18,069.2	18,036.6
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,422.9	3,101.4
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,099.4	3,923.2
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,717.0	33,734.3
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,497.2	$ 3,146.0