Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information
Premium Income	$ 2,610.3	$ 2,459.3
Operating Segments
Segment Reporting Information
Premium Income	2,610.3	2,459.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,707.4	1,609.6
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	516.7	504.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	263.1	240.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	442.6	413.1
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	45.8	43.5
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	222.9	214.5
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	142.0	124.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	74.3	69.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	231.7	188.6
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	103.5	91.7
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	48.7	39.2
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	43.1	31.8
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	36.4	25.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	446.9	429.5
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	243.2	235.7
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	114.3	105.3
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.4	88.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	224.3	231.6
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	174.5	175.1
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 49.8	$ 56.5