Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information
Premium Income	$ 2,371.4	$ 2,338.7
Operating Segments
Segment Reporting Information
Premium Income	2,371.4	2,338.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,527.7	1,501.9
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	463.0	452.9
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	203.2	188.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	414.5	414.4
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	41.7	41.0
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	109.5	110.7
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	230.4	234.4
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	65.4	59.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income	164.6	154.0
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	90.8	87.9
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	30.9	27.2
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	23.9	21.7
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	19.0	17.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	434.7	419.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	249.3	242.2
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	93.8	87.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	91.6	89.5
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	244.4	263.5
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	77.0	98.1
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	164.8	163.0
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.6	$ 2.4