Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information
Premium Income	$ 2,627.2	$ 2,509.1	$ 5,237.5	$ 4,968.4
Operating Segments
Segment Reporting Information
Premium Income	2,627.2	2,509.1	5,237.5	4,968.4
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,730.9	1,641.4	3,438.3	3,251.0
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	521.5	516.1	1,038.2	1,020.8
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	276.3	256.2	539.4	496.5
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	447.5	418.2	890.1	831.3
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	46.4	43.6	92.2	87.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	222.9	213.4	445.8	427.9
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	142.2	125.8	284.2	250.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	74.1	68.1	148.4	137.4
Operating Segments | Unum International
Segment Reporting Information
Premium Income	228.8	207.9	460.5	396.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	102.3	104.7	205.8	196.4
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	48.8	41.1	97.5	80.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	40.6	33.5	83.7	65.3
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	37.1	28.6	73.5	54.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	446.2	430.6	893.1	860.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	241.7	235.6	484.9	471.3
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	115.2	106.6	229.5	211.9
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.3	88.4	178.7	176.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	221.3	229.2	445.6	460.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	173.3	174.0	347.8	349.1
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 48.0	$ 55.2	$ 97.8	$ 111.7