Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information
Premium Income	$ 2,087.5	$ 2,006.3
Net Investment Income	606.4	602.0
Other Income	52.0	54.4
Operating Segments
Segment Reporting Information
Premium Income	2,087.5	2,006.3
Net Investment Income	606.4	602.0
Other Income	52.0	54.4
Operating Revenue	2,745.9	2,662.7
Operating Income (Loss)	328.8	321.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,301.3	1,230.2
Net Investment Income	207.4	215.0
Other Income	29.0	30.9
Operating Revenue	1,537.7	1,476.1
Operating Income (Loss)	215.9	214.3
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.3	139.0
Net Investment Income	26.8	23.5
Other Income	0.0	0.0
Operating Revenue	166.1	162.5
Operating Income (Loss)	33.6	32.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	351.2	332.2
Net Investment Income	34.0	37.0
Other Income	0.3	0.0
Operating Revenue	385.5	369.2
Operating Income (Loss)	77.4	77.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	295.7	304.9
Net Investment Income	333.4	320.4
Other Income	22.4	23.2
Operating Revenue	651.5	648.5
Operating Income (Loss)	33.7	26.7
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	4.8	6.1
Other Income	0.3	0.3
Operating Revenue	5.1	6.4
Operating Income (Loss)	$ (31.8)	$ (29.9)