Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information
Premium Income	$ 2,417.3	$ 2,374.4	$ 4,820.6	$ 4,752.7
Operating Segments
Segment Reporting Information
Premium Income	2,417.3	2,374.4	4,820.6	4,752.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,572.3	1,522.1	3,117.2	3,047.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	474.4	458.6	938.3	916.3
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	232.1	213.6	453.7	428.8
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	419.6	414.6	832.2	824.6
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	43.8	42.0	85.9	83.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	215.7	212.1	436.2	430.8
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	117.8	113.6	231.4	229.3
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	68.9	67.6	139.5	134.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	179.4	183.5	367.2	357.9
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	94.3	105.1	197.7	202.2
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	33.3	28.0	65.5	55.3
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	29.4	27.9	58.4	55.9
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	22.4	22.5	45.6	44.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	427.6	419.7	858.3	846.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	238.2	236.4	477.9	477.1
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	101.3	96.1	203.0	192.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	88.1	87.2	177.4	176.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	238.0	249.1	477.9	500.8
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	62.4	72.4	125.8	144.5
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	173.7	174.9	348.5	352.3
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.9	$ 1.8	$ 3.6	$ 4.0