Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information
Premium Income	$ 2,391.7	$ 2,353.7	$ 7,212.3	$ 7,106.4
Operating Segments
Segment Reporting Information
Premium Income	2,391.7	2,353.7	7,212.3	7,106.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,559.6	1,500.8	4,676.8	4,548.7
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	480.9	451.4	1,419.2	1,367.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	232.2	212.4	685.9	641.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	417.7	404.2	1,249.9	1,228.8
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	43.8	39.6	129.7	123.0
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	199.0	209.6	635.2	640.4
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	118.9	115.7	350.3	345.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	67.1	67.9	206.6	202.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income	173.3	181.6	540.5	539.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	89.8	101.6	287.5	303.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	35.0	29.1	100.5	84.4
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	27.4	28.3	85.8	84.2
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	21.1	22.6	66.7	67.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	423.3	420.8	1,281.6	1,266.9
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	236.1	238.0	714.0	715.1
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	100.2	94.6	303.2	287.3
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	87.0	88.2	264.4	264.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	235.5	250.5	713.4	751.3
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	59.5	72.3	185.3	216.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	174.5	176.1	523.0	528.4
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.5	$ 2.1	$ 5.1	$ 6.1