Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Assets	$ 61,967.9	$ 60,563.6
Operating Segments
Segment Reporting Information
Assets	61,967.9	60,563.6
Operating Segments | Unum US
Segment Reporting Information
Assets	17,963.4	18,242.4
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,453.7	3,432.4
Operating Segments | Colonial Life
Segment Reporting Information
Assets	3,759.3	3,776.8
Operating Segments | Closed Block
Segment Reporting Information
Assets	33,549.7	32,992.4
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 3,241.8	$ 2,119.6