Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information
Premium Income	$ 2,374.4	$ 2,368.7	$ 4,752.7	$ 4,740.1
Operating Segments
Segment Reporting Information
Premium Income	2,374.4	2,368.7	4,752.7	4,740.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,522.1	1,522.7	3,047.9	3,050.4
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	458.6	461.0	916.3	924.0
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	213.6	203.6	428.8	406.8
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	414.6	414.6	824.6	829.1
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	42.0	41.7	83.4	83.4
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	113.6	113.1	229.3	222.6
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	212.1	224.0	430.8	454.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	67.6	64.7	134.7	130.1
Operating Segments | Unum International
Segment Reporting Information
Premium Income	183.5	157.1	357.9	321.7
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	105.1	89.0	202.2	179.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	28.0	25.1	55.3	56.0
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	27.9	24.6	55.9	48.5
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	22.5	18.4	44.5	37.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.7	438.6	846.1	873.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	236.4	250.3	477.1	499.6
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	96.1	96.8	192.7	190.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	87.2	91.5	176.3	183.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	249.1	250.3	500.8	494.7
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	72.4	83.1	144.5	160.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	174.9	165.7	352.3	330.5
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.8	$ 1.5	$ 4.0	$ 4.1