Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information
Premium Income	$ 2,353.7	$ 2,318.1	$ 7,106.4	$ 7,058.2
Operating Segments
Segment Reporting Information
Premium Income	2,353.7	2,318.1	7,106.4	7,058.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,500.8	1,483.4	4,548.7	4,533.8
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	451.4	450.5	1,367.7	1,374.5
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	212.4	196.2	641.2	603.0
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	404.2	406.3	1,228.8	1,235.4
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	39.6	39.7	123.0	123.1
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	115.7	117.7	345.0	340.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	209.6	212.2	640.4	666.6
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	67.9	60.8	202.6	190.9
Operating Segments | Unum International
Segment Reporting Information
Premium Income	181.6	165.4	539.5	487.1
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	101.6	92.2	303.8	272.0
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	29.1	27.3	84.4	83.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	28.3	25.4	84.2	73.9
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	22.6	20.5	67.1	57.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	420.8	419.9	1,266.9	1,293.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	238.0	238.9	715.1	738.4
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	94.6	91.9	287.3	282.5
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.2	89.1	264.5	272.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	250.5	249.4	751.3	744.1
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	72.3	80.0	216.8	240.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	176.1	167.7	528.4	498.2
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.1	$ 1.7	$ 6.1	$ 5.8