Premium Income by Major Line of Business within Each 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
Operating Segments
Segment Reporting Information
Premium Income	2,142.9	2,087.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,360.4	1,301.3
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	434.4	430.6
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	157.9	157.5
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	367.8	351.5
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	36.6	34.6
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	107.0	123.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	215.2	203.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	41.5	0.0
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	121.3	139.3
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	80.8	93.1
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	24.8	28.7
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	15.7	17.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	374.3	351.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	219.1	205.6
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	74.2	67.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	81.0	77.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	286.9	295.7
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	121.3	133.6
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	163.1	161.7
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.5	$ 0.4