Assets by Segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information
Total Assets	$ 62,497.1	$ 59,403.6
Operating Segments
Segment Reporting Information
Total Assets	62,497.1	59,403.6
Operating Segments | Unum U.S.
Segment Reporting Information
Total Assets	18,676.5	18,384.3
Operating Segments | Unum U.K.
Segment Reporting Information
Total Assets	3,702.5	3,654.1
Operating Segments | Colonial Life
Segment Reporting Information
Total Assets	3,692.2	3,482.9
Operating Segments | Closed Block
Segment Reporting Information
Total Assets	33,960.2	31,564.2
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Total Assets	$ 2,465.7	$ 2,318.1