Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information
Premium Income	$ 10,046.0	$ 9,616.5	$ 9,475.0
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	7,398.5	7,060.8	7,072.9
Life
Segment Reporting Information
Premium Income	2,647.5	2,555.7	2,402.1
Operating Segments
Segment Reporting Information
Premium Income	10,046.0	9,616.5	9,475.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,579.2	6,251.4	6,072.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	2,057.2	1,911.7	1,827.8
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	1,012.3	926.3	864.0
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	1,679.0	1,669.1	1,641.9
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	175.5	173.7	165.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	850.1	833.7	840.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	527.0	461.1	459.8
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	278.1	275.8	272.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	825.2	718.8	717.0
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	396.1	376.9	401.9
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	169.3	138.2	112.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	141.5	114.0	112.6
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	118.3	89.7	90.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,726.1	1,702.0	1,690.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	946.1	948.9	953.3
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	426.5	401.1	384.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	353.5	352.0	352.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	915.5	944.3	995.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	696.0	697.4	704.3
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 219.5	$ 246.9	$ 291.5