Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Premium Income	$ 2,319.9	$ 2,318.1	$ 2,368.7	$ 2,371.4	$ 2,352.6	$ 2,331.2	$ 2,343.1	$ 2,338.7	$ 9,378.1	$ 9,365.6	$ 8,986.1
Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									6,975.2	6,944.9	6,673.5
Life
Segment Reporting Information
Premium Income									2,402.9	2,420.7	2,312.6
Operating Segments
Segment Reporting Information
Premium Income									9,378.1	9,365.6	8,986.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income									6,018.9	6,016.6	5,736.4
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income									1,828.5	1,823.1	1,766.2
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income									799.2	768.8	706.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income									1,640.5	1,662.0	1,583.7
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income									163.9	165.7	156.3
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income									456.0	440.7	425.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income									875.2	910.2	895.7
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income									255.6	246.1	202.8
Operating Segments | Unum International
Segment Reporting Information
Premium Income									652.8	630.5	568.8
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income									364.9	353.4	358.9
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income									108.5	115.7	110.8
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income									99.8	89.5	81.7
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income									79.6	71.9	17.4
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,712.0	1,685.0	1,603.8
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income									975.1	973.4	929.3
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income									376.4	351.6	328.4
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									360.5	360.0	346.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									994.4	1,033.5	1,077.1
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income									319.6	374.3	420.8
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									666.9	651.6	648.3
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 7.9	$ 7.6	$ 8.0