Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information
Premium Income	$ 2,142.2	$ 2,081.6	$ 4,285.1	$ 4,169.1
Operating Segments
Segment Reporting Information
Premium Income	2,142.2	2,081.6	4,285.1	4,169.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,357.7	1,299.4	2,718.1	2,600.7
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	437.7	432.9	872.1	863.5
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	158.3	156.2	316.2	313.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	363.9	348.7	731.7	700.2
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	36.7	34.9	73.3	69.5
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	106.7	124.9	213.7	248.8
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	213.2	201.8	428.4	405.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	41.2	0.0	82.7	0.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	126.8	140.8	248.1	280.1
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	83.8	95.5	164.6	188.6
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	25.4	27.3	50.2	56.0
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	17.6	18.0	33.3	35.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	376.3	351.4	750.6	702.6
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	220.1	205.9	439.2	411.5
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	75.0	67.7	149.2	135.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	81.2	77.8	162.2	155.7
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	281.4	290.0	568.3	585.7
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	118.5	130.5	239.8	264.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	160.9	158.9	324.0	320.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.0	$ 0.6	$ 4.5	$ 1.0