Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Segment Reporting Information
Assets	$ 63,014.5	$ 62,450.2
Operating Segments
Segment Reporting Information
Assets	63,014.5	62,450.2
Operating Segments | Unum US
Segment Reporting Information
Assets	18,865.0	18,676.5
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,613.4	3,702.5
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,774.7	3,692.2
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,462.1	33,960.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,299.3	$ 2,418.8