Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Assets	$ 63,012.3	$ 64,013.1
Operating Segments
Segment Reporting Information
Assets	63,012.3	64,013.1
Operating Segments | Unum US
Segment Reporting Information
Assets	18,036.3	18,109.1
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,497.6	3,428.1
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,292.9	4,184.1
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,603.8	35,051.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,581.7	$ 3,240.6