Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information
Premium Income	$ 10,831.0	$ 10,497.4	$ 10,046.0
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	7,756.4	7,640.2	7,398.5
Life
Segment Reporting Information
Premium Income	3,074.6	2,857.2	2,647.5
Operating Segments
Segment Reporting Information
Premium Income	10,831.0	10,497.4	10,046.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	7,083.5	6,883.2	6,579.2
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	2,011.1	2,086.1	2,057.2
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	1,138.4	1,084.0	1,012.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	1,871.1	1,784.7	1,679.0
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	195.6	186.1	175.5
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	927.4	879.2	850.1
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	615.1	566.0	527.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	324.8	297.1	278.1
Operating Segments | Unum International
Segment Reporting Information
Premium Income	1,082.8	949.5	825.2
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	427.5	418.0	396.1
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	275.7	211.3	169.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	184.2	165.6	141.5
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	195.4	154.6	118.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	1,839.1	1,783.9	1,726.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	993.5	969.5	946.1
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	483.6	458.0	426.5
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	362.0	356.4	353.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	825.6	880.8	915.5
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	670.8	696.1	696.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 154.8	$ 184.7	$ 219.5