Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information
Premium Income	$ 2,331.2	$ 2,251.8	$ 7,013.0	$ 6,722.8
Operating Segments
Segment Reporting Information
Premium Income	2,331.2	2,251.8	7,013.0	6,722.8
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,501.9	1,446.2	4,508.3	4,301.3
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	453.6	444.1	1,363.7	1,322.7
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	194.4	180.4	570.7	527.7
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	415.0	399.3	1,249.4	1,189.6
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	41.5	39.6	124.2	116.9
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	109.2	109.5	328.4	319.1
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	226.2	222.3	689.2	676.0
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	62.0	51.0	182.7	149.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	152.3	138.0	464.9	416.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	84.8	91.3	262.3	271.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	27.9	27.1	83.9	83.6
Operating Segments | Unum International | Supplemental Insurance
Segment Reporting Information
Premium Income	39.6	19.6	118.7	61.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.9	400.0	1,260.1	1,193.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	243.0	231.9	727.6	691.8
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	86.7	81.4	262.6	243.9
Operating Segments | Colonial Life | Cancer and Critical Illness
Segment Reporting Information
Premium Income	90.2	86.7	269.9	258.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	257.1	267.6	779.7	811.3
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	91.4	102.7	283.9	319.0
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	163.6	163.0	489.2	485.8
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.1	$ 1.9	$ 6.6	$ 6.5