Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information
Premium Income	$ 2,688.0	$ 2,628.8	$ 8,138.9	$ 7,866.3
Operating Segments
Segment Reporting Information
Premium Income	2,688.0	2,628.8	8,138.9	7,866.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,755.4	1,723.5	5,334.9	5,161.8
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	499.2	522.1	1,511.5	1,560.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	286.0	271.3	853.6	810.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	468.3	447.8	1,404.5	1,337.9
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	48.9	47.1	146.3	139.3
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	231.6	219.3	700.2	665.1
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	140.0	140.4	475.4	424.6
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	81.4	75.5	243.4	223.9
Operating Segments | Unum International
Segment Reporting Information
Premium Income	281.1	246.6	798.9	707.1
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	111.3	106.6	319.4	312.4
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	71.9	58.8	201.6	156.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	47.4	41.4	136.3	125.1
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	50.5	39.8	141.6	113.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	456.5	441.9	1,375.9	1,335.0
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	247.5	240.6	743.7	725.5
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	118.7	113.1	360.7	342.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	90.3	88.2	271.5	266.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	195.0	216.8	629.2	662.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	158.6	173.7	510.7	521.5
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 36.4	$ 43.1	$ 118.5	$ 140.9