Assets by Segment (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Segment Reporting Information
Assets	$ 63,853.1	$ 60,563.6
Operating Segments
Segment Reporting Information
Assets	63,853.1	60,563.6
Operating Segments | Unum US
Segment Reporting Information
Assets	18,310.8	18,242.4
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,325.2	3,432.4
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,874.2	3,776.8
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,559.8	33,000.9
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,783.1	$ 2,111.1