Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Premium Income	$ 2,250.0	$ 2,142.9
Operating Segments
Segment Reporting Information
Premium Income	2,250.0	2,142.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,438.8	1,360.4
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	441.2	434.4
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	176.3	157.9
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	399.2	367.8
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	38.7	36.6
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	104.9	107.0
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	229.8	215.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	48.7	41.5
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.6	121.3
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	90.8	80.8
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	28.6	24.8
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	20.2	15.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	398.3	374.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	231.3	219.1
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	81.0	74.2
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	86.0	81.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	273.3	286.9
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	109.4	121.3
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	161.3	163.1
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.6	$ 2.5