Assets by Segment (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Segment Reporting Information
Assets	$ 61,503.8	$ 62,450.2
Operating Segments
Segment Reporting Information
Assets	61,503.8	62,450.2
Operating Segments | Unum US
Segment Reporting Information
Assets	18,630.7	18,676.5
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,636.3	3,702.5
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,770.3	3,692.2
Operating Segments | Closed Block
Segment Reporting Information
Assets	33,431.9	33,960.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,034.6	$ 2,418.8