Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Premium Income	$ 2,338.7	$ 2,250.0
Operating Segments
Segment Reporting Information
Premium Income	2,338.7	2,250.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,501.9	1,438.8
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	452.9	441.2
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	188.7	176.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	414.4	399.2
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	41.0	38.7
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	110.7	104.9
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	234.4	229.8
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	59.8	48.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income	154.0	139.6
Operating Segments | Unum International | Group Long term Disability
Segment Reporting Information
Premium Income	87.9	90.8
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	27.2	28.6
Operating Segments | Unum International | Supplemental Insurance
Segment Reporting Information
Premium Income	38.9	20.2
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	419.3	398.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	242.2	231.3
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	87.6	81.0
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	89.5	86.0
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	263.5	273.3
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	98.1	109.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	163.0	161.3
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.4	$ 2.6