Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information
Premium Income	$ 2,368.7	$ 2,343.1	$ 4,740.1	$ 4,681.8
Operating Segments
Segment Reporting Information
Premium Income	2,368.7	2,343.1	4,740.1	4,681.8
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,522.7	1,504.5	3,050.4	3,006.4
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	461.0	457.2	924.0	910.1
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	203.6	187.6	406.8	376.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	414.6	420.0	829.1	834.4
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	41.7	41.7	83.4	82.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	113.1	108.5	222.6	219.2
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	224.0	228.6	454.4	463.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	64.7	60.9	130.1	120.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	157.1	158.6	321.7	312.6
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	89.0	89.6	179.8	177.5
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	25.1	28.8	56.0	56.0
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	24.6	22.4	48.5	44.1
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	18.4	17.8	37.4	35.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	438.6	420.9	873.3	840.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	250.3	242.4	499.6	484.6
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	96.8	88.3	190.6	175.9
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	91.5	90.2	183.1	179.7
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	250.3	259.1	494.7	522.6
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	83.1	94.4	160.1	192.5
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	165.7	162.6	330.5	325.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.5	$ 2.1	$ 4.1	$ 4.5