Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information
Premium Income	$ 10,497.4	$ 10,046.0	$ 9,616.5
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	7,640.2	7,398.5	7,060.8
Life
Segment Reporting Information
Premium Income	2,857.2	2,647.5	2,555.7
Operating Segments
Segment Reporting Information
Premium Income	10,497.4	10,046.0	9,616.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	6,883.2	6,579.2	6,251.4
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	2,086.1	2,057.2	1,911.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	1,084.0	1,012.3	926.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	1,784.7	1,679.0	1,669.1
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	186.1	175.5	173.7
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	879.2	850.1	833.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	566.0	527.0	461.1
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	297.1	278.1	275.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income	949.5	825.2	718.8
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	418.0	396.1	376.9
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	211.3	169.3	138.2
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	165.6	141.5	114.0
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	154.6	118.3	89.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,783.9	1,726.1	1,702.0
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	969.5	946.1	948.9
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	458.0	426.5	401.1
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	356.4	353.5	352.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	880.8	915.5	944.3
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	696.1	696.0	697.4
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 184.7	$ 219.5	$ 246.9