Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Assets	$ 62,524.7	$ 61,941.5
Operating Segments
Segment Reporting Information
Assets	62,524.7	61,941.5
Operating Segments | Unum US
Segment Reporting Information
Assets	18,111.9	18,036.6
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,208.9	3,101.4
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,976.8	3,923.2
Operating Segments | Closed Block
Segment Reporting Information
Assets	33,882.2	33,734.3
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,344.9	$ 3,146.0