Premium Income by Major Line of Business within Each 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
Operating Segments
Segment Reporting Information
Premium Income	2,087.5	2,006.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,301.3	1,230.2
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	430.6	405.3
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	157.5	147.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	351.5	335.0
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	34.6	32.6
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	123.9	117.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	203.2	191.7
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.3	139.0
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	93.1	96.5
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	28.7	30.5
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	17.5	12.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	351.2	332.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	205.6	196.5
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	67.7	62.3
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	77.9	73.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	295.7	304.9
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	133.6	146.5
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	161.7	158.2
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 0.4	$ 0.2