Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information
Assets	$ 62,249.8	$ 64,013.1
Operating Segments
Segment Reporting Information
Assets	62,249.8	64,013.1
Operating Segments | Unum US
Segment Reporting Information
Assets	17,297.1	18,109.1
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,211.3	3,428.1
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,145.2	4,184.1
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,360.5	35,051.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,235.7	$ 3,240.6