Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting
Premium Income	$ 2,817.8	$ 2,748.0	$ 5,611.8	$ 5,450.9
Operating Segments
Segment Reporting
Premium Income	2,817.8	2,748.0	5,611.8	5,450.9
Operating Segments | Unum US
Segment Reporting
Premium Income	1,858.2	1,798.6	3,698.7	3,579.5
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting
Premium Income	495.3	507.8	994.1	1,012.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting
Premium Income	332.2	289.3	642.9	567.6
Operating Segments | Unum US | Group Life
Segment Reporting
Premium Income	499.7	470.0	995.1	936.2
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting
Premium Income	53.8	49.2	106.8	97.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting
Premium Income	251.7	234.5	505.5	468.6
Operating Segments | Unum US | Individual Disability
Segment Reporting
Premium Income	142.8	166.7	289.3	335.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting
Premium Income	82.7	81.1	165.0	162.0
Operating Segments | Unum International
Segment Reporting
Premium Income	289.3	271.1	576.0	517.8
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting
Premium Income	107.5	107.9	212.3	208.1
Operating Segments | Unum International | Group Life
Segment Reporting
Premium Income	77.2	68.1	154.2	129.7
Operating Segments | Unum International | Supplemental
Segment Reporting
Premium Income	50.8	47.0	101.1	88.9
Operating Segments | Unum International | Unum Poland
Segment Reporting
Premium Income	53.8	48.1	108.4	91.1
Operating Segments | Colonial Life
Segment Reporting
Premium Income	477.4	462.1	950.1	919.4
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting
Premium Income	254.5	249.1	507.3	496.2
Operating Segments | Colonial Life | Life
Segment Reporting
Premium Income	131.1	122.1	259.4	242.0
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting
Premium Income	91.8	90.9	183.4	181.2
Operating Segments | Closed Block
Segment Reporting
Premium Income	192.9	216.2	387.0	434.2
Operating Segments | Closed Block | Long-term Care
Segment Reporting
Premium Income	159.4	175.9	318.8	352.1
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting
Premium Income	$ 33.5	$ 40.3	$ 68.2	$ 82.1