Premium Income by Major Line of Business within Each 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	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	227.2	202.8	443.1	417.1
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	432.9	409.4	863.5	814.7
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	156.2	151.6	313.7	299.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	348.7	336.0	700.2	671.0
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	34.9	33.3	69.5	65.9
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	124.9	118.4	248.8	236.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	201.8	187.7	405.0	379.4
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	36.9	38.3	70.5	70.9
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	95.5	99.4	188.6	195.9
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	27.3	30.4	56.0	60.9
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	18.0	12.4	35.5	24.4
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	77.9	77.6	155.3	155.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	205.9	198.1	411.5	394.6
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	67.7	63.3	135.4	125.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	77.8	74.5	155.7	147.9
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	32.6	36.6	66.3	63.3
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	130.5	144.3	264.1	290.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.9	158.2	320.6	316.4
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.6	$ 0.5	$ 1.0	$ 0.7