Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information
Premium Income	$ 2,628.8	$ 2,525.9	$ 7,866.3	$ 7,494.3
Operating Segments
Segment Reporting Information
Premium Income	2,628.8	2,525.9	7,866.3	7,494.3
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,723.5	1,657.7	5,161.8	4,908.7
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	522.1	518.5	1,560.3	1,539.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	271.3	256.5	810.7	753.0
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	447.8	423.0	1,337.9	1,254.3
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	47.1	44.1	139.3	131.2
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	219.3	209.5	665.1	637.4
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	140.4	136.7	424.6	386.7
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	75.5	69.4	223.9	206.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income	246.6	210.6	707.1	607.1
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	106.6	101.1	312.4	297.5
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	58.8	44.2	156.3	124.5
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	41.4	34.6	125.1	99.9
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	39.8	30.7	113.3	85.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	441.9	431.2	1,335.0	1,291.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	240.6	236.7	725.5	708.0
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	113.1	106.2	342.6	318.1
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.2	88.3	266.9	265.2
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	216.8	226.4	662.4	687.2
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	173.7	172.1	521.5	521.2
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 43.1	$ 54.3	$ 140.9	$ 166.0