Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting Information
Assets	$ 63,381.6	$ 61,941.5
Operating Segments
Segment Reporting Information
Assets	63,381.6	61,941.5
Operating Segments | Unum US
Segment Reporting Information
Assets	18,232.8	18,036.6
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,358.1	3,101.4
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,039.8	3,923.2
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,466.6	33,734.3
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,284.3	$ 3,146.0