Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information
Premium Income	$ 2,748.0	$ 2,627.2	$ 5,450.9	$ 5,237.5
Operating Segments
Segment Reporting Information
Premium Income	2,748.0	2,627.2	5,450.9	5,237.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,798.6	1,730.9	3,579.5	3,438.3
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	507.8	521.5	1,012.3	1,038.2
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	289.3	276.3	567.6	539.4
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	470.0	447.5	936.2	890.1
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	49.2	46.4	97.4	92.2
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	234.5	222.9	468.6	445.8
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	166.7	142.2	335.4	284.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	81.1	74.1	162.0	148.4
Operating Segments | Unum International
Segment Reporting Information
Premium Income	271.1	228.8	517.8	460.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	107.9	102.3	208.1	205.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	68.1	48.8	129.7	97.5
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	47.0	40.6	88.9	83.7
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	48.1	37.1	91.1	73.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	462.1	446.2	919.4	893.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	249.1	241.7	496.2	484.9
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	122.1	115.2	242.0	229.5
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	90.9	89.3	181.2	178.7
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	216.2	221.3	434.2	445.6
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	175.9	173.3	352.1	347.8
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 40.3	$ 48.0	$ 82.1	$ 97.8