Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information
Premium Income	$ 2,702.9	$ 2,610.3
Operating Segments
Segment Reporting Information
Premium Income	2,702.9	2,610.3
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	1,780.9	1,707.4
Operating Segments \| Unum US \| Group Long-term Disability
Segment Reporting Information
Premium Income	504.5	516.7
Operating Segments \| Unum US \| Group Short-term Disability
Segment Reporting Information
Premium Income	278.3	263.1
Operating Segments \| Unum US \| Group Life
Segment Reporting Information
Premium Income	466.2	442.6
Operating Segments \| Unum US \| Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	48.2	45.8
Operating Segments \| Unum US \| Voluntary Benefits
Segment Reporting Information
Premium Income	234.1	222.9
Operating Segments \| Unum US \| Individual Disability
Segment Reporting Information
Premium Income	168.7	142.0
Operating Segments \| Unum US \| Dental and Vision
Segment Reporting Information
Premium Income	80.9	74.3
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	246.7	231.7
Operating Segments \| Unum International \| Group Long-term Disability
Segment Reporting Information
Premium Income	100.2	103.5
Operating Segments \| Unum International \| Group Life
Segment Reporting Information
Premium Income	61.6	48.7
Operating Segments \| Unum International \| Supplemental
Segment Reporting Information
Premium Income	41.9	43.1
Operating Segments \| Unum International \| Unum Poland
Segment Reporting Information
Premium Income	43.0	36.4
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	457.3	446.9
Operating Segments \| Colonial Life \| Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	247.1	243.2
Operating Segments \| Colonial Life \| Life
Segment Reporting Information
Premium Income	119.9	114.3
Operating Segments \| Colonial Life \| Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	90.3	89.4
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	218.0	224.3
Operating Segments \| Closed Block \| Long-term Care
Segment Reporting Information
Premium Income	176.2	174.5
Operating Segments \| Closed Block \| Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 41.8	$ 49.8