Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting
Premium Income	$ 2,794.0	$ 2,702.9
Operating Segments
Segment Reporting
Premium Income	2,794.0	2,702.9
Operating Segments | Unum US
Segment Reporting
Premium Income	1,840.5	1,780.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting
Premium Income	498.8	504.5
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting
Premium Income	310.7	278.3
Operating Segments | Unum US | Group Life
Segment Reporting
Premium Income	495.4	466.2
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting
Premium Income	53.0	48.2
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting
Premium Income	253.8	234.1
Operating Segments | Unum US | Individual Disability
Segment Reporting
Premium Income	146.5	168.7
Operating Segments | Unum US | Dental and Vision
Segment Reporting
Premium Income	82.3	80.9
Operating Segments | Unum International
Segment Reporting
Premium Income	286.7	246.7
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting
Premium Income	104.8	100.2
Operating Segments | Unum International | Group Life
Segment Reporting
Premium Income	77.0	61.6
Operating Segments | Unum International | Supplemental
Segment Reporting
Premium Income	50.3	41.9
Operating Segments | Unum International | Unum Poland
Segment Reporting
Premium Income	54.6	43.0
Operating Segments | Colonial Life
Segment Reporting
Premium Income	472.7	457.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting
Premium Income	252.8	247.1
Operating Segments | Colonial Life | Life
Segment Reporting
Premium Income	128.3	119.9
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting
Premium Income	91.6	90.3
Operating Segments | Closed Block
Segment Reporting
Premium Income	194.1	218.0
Operating Segments | Closed Block | Long-term Care
Segment Reporting
Premium Income	159.4	176.2
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting
Premium Income	$ 34.7	$ 41.8