Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information
Premium Income	$ 9,623.4	$ 9,481.0	$ 9,378.1
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	7,060.8	7,072.9	6,975.2
Life
Segment Reporting Information
Premium Income	2,562.6	2,408.1	2,402.9
Operating Segments
Segment Reporting Information
Premium Income	9,623.4	9,481.0	9,378.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,258.3	6,078.0	6,018.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	1,911.7	1,827.8	1,828.5
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	926.3	864.0	799.2
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	1,669.1	1,641.9	1,640.5
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	173.7	165.1	163.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	840.6	846.7	875.2
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	461.1	459.8	456.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	275.8	272.7	255.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income	718.8	717.0	652.8
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	376.9	401.9	364.9
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	138.2	112.3	108.5
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	114.0	112.6	99.8
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	89.7	90.2	79.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,702.0	1,690.2	1,712.0
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	948.9	953.3	975.1
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	401.1	384.7	376.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	352.0	352.2	360.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	944.3	995.8	994.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	697.4	704.3	666.9
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 246.9	$ 291.5	$ 327.5