Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Premium Income	$ 9,481.0	$ 9,378.1	$ 9,365.6
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	7,072.9	6,975.2	6,944.9
Life
Segment Reporting Information
Premium Income	2,408.1	2,402.9	2,420.7
Operating Segments
Segment Reporting Information
Premium Income	9,481.0	9,378.1	9,365.6
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,078.0	6,018.9	6,016.6
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	1,827.8	1,828.5	1,823.1
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	864.0	799.2	768.8
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	1,641.9	1,640.5	1,662.0
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	165.1	163.9	165.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	459.8	456.0	440.7
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	846.7	875.2	910.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	272.7	255.6	246.1
Operating Segments | Unum International
Segment Reporting Information
Premium Income	717.0	652.8	630.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	401.9	364.9	353.4
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	112.3	108.5	115.7
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	112.6	99.8	89.5
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	90.2	79.6	71.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,690.2	1,712.0	1,685.0
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	953.3	975.1	973.4
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	384.7	376.4	351.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	352.2	360.5	360.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	995.8	994.4	1,033.5
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	284.0	319.6	374.3
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	704.3	666.9	651.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 7.5	$ 7.9	$ 7.6