Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information
Premium Income	$ 2,081.6	$ 2,017.5	$ 4,169.1	$ 4,023.8
Net Investment Income	623.3	630.7	1,229.7	1,232.7
Other Income	51.1	54.7	103.1	109.1
Operating Segments
Segment Reporting Information
Premium Income	2,081.6	2,017.5	4,169.1	4,023.8
Net Investment Income	623.3	630.7	1,229.7	1,232.7
Other Income	51.1	54.7	103.1	109.1
Operating Revenue	2,756.0	2,702.9	5,501.9	5,365.6
Operating Income (Loss)	338.9	321.9	667.7	643.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,299.4	1,236.4	2,600.7	2,466.6
Net Investment Income	207.7	215.7	415.1	430.7
Other Income	27.9	31.4	56.9	62.3
Operating Revenue	1,535.0	1,483.5	3,072.7	2,959.6
Operating Income (Loss)	227.2	202.8	443.1	417.1
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	140.8	142.2	280.1	281.2
Net Investment Income	34.0	38.6	60.8	62.1
Other Income	0.2	0.0	0.2	0.0
Operating Revenue	175.0	180.8	341.1	343.3
Operating Income (Loss)	36.9	38.3	70.5	70.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	351.4	335.9	702.6	668.1
Net Investment Income	35.6	36.9	69.6	73.9
Other Income	0.3	0.0	0.6	0.0
Operating Revenue	387.3	372.8	772.8	742.0
Operating Income (Loss)	77.9	77.6	155.3	155.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	290.0	303.0	585.7	607.9
Net Investment Income	341.1	331.7	674.5	652.1
Other Income	21.4	22.8	43.8	46.0
Operating Revenue	652.5	657.5	1,304.0	1,306.0
Operating Income (Loss)	32.6	36.6	66.3	63.3
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0.0	0.0	0.0	0.0
Net Investment Income	4.9	7.8	9.7	13.9
Other Income	1.3	0.5	1.6	0.8
Operating Revenue	6.2	8.3	11.3	14.7
Operating Income (Loss)	$ (35.7)	$ (33.4)	$ (67.5)	$ (63.3)