Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Premium Income	$ 2,318.1	$ 2,331.2	$ 7,058.2	$ 7,013.0
Operating Segments
Segment Reporting Information
Premium Income	2,318.1	2,331.2	7,058.2	7,013.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,483.4	1,501.9	4,533.8	4,508.3
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	450.5	453.6	1,374.5	1,363.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	196.2	194.4	603.0	570.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	406.3	415.0	1,235.4	1,249.4
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	39.7	41.5	123.1	124.2
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	117.7	109.2	340.3	328.4
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	212.2	226.2	666.6	689.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	60.8	62.0	190.9	182.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	165.4	152.3	487.1	464.9
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	92.2	84.8	272.0	262.3
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	27.3	27.9	83.3	83.9
Operating Segments | Unum International | Supplemental
Segment Reporting Information
Premium Income	25.4	21.6	73.9	65.7
Operating Segments | Unum International | Unum Poland
Segment Reporting Information
Premium Income	20.5	18.0	57.9	53.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.9	419.9	1,293.2	1,260.1
Operating Segments | Colonial Life | Accident, Sickness, and Disability
Segment Reporting Information
Premium Income	238.9	243.0	738.4	727.6
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income	91.9	86.7	282.5	262.6
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.1	90.2	272.3	269.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	249.4	257.1	744.1	779.7
Operating Segments | Closed Block | Individual Disability
Segment Reporting Information
Premium Income	80.0	91.4	240.1	283.9
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	167.7	163.6	498.2	489.2
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 1.7	$ 2.1	$ 5.8	$ 6.6