Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information
Premium Income	$ 2,142.9	$ 2,087.5
Net Investment Income	602.4	606.4
Other Income	50.2	52.0
Operating Segments
Segment Reporting Information
Premium Income	2,142.9	2,087.5
Net Investment Income	602.4	606.4
Other Income	50.2	52.0
Operating Revenue	2,795.5	2,745.9
Operating Income (Loss)	339.9	324.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,360.4	1,301.3
Net Investment Income	202.5	207.4
Other Income	28.7	29.0
Operating Revenue	1,591.6	1,537.7
Operating Income (Loss)	239.1	215.9
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	121.3	139.3
Net Investment Income	26.6	26.8
Other Income	0.0	0.0
Operating Revenue	147.9	166.1
Operating Income (Loss)	26.6	33.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	374.3	351.2
Net Investment Income	35.1	34.0
Other Income	0.3	0.3
Operating Revenue	409.7	385.5
Operating Income (Loss)	82.4	77.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	286.9	295.7
Net Investment Income	335.3	333.4
Other Income	20.8	22.4
Operating Revenue	643.0	651.5
Operating Income (Loss)	31.6	33.7
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0
Net Investment Income	2.9	4.8
Other Income	0.4	0.3
Operating Revenue	3.3	5.1
Operating Income (Loss)	$ (39.8)	$ (35.9)