Assets by Segment (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information
Total Assets	$ 60,589.7	$ 62,450.2
Operating Segments
Segment Reporting Information
Total Assets	60,589.7	62,450.2
Operating Segments | Unum U.S.
Segment Reporting Information
Total Assets	18,242.4	18,676.5
Operating Segments | Unum U.K.
Segment Reporting Information
Total Assets	3,432.4	3,702.5
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	3,776.8	3,692.2
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	32,992.4	33,960.2
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 2,145.7	$ 2,418.8