Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information
Premium Income	$ 2,343.1	$ 2,221.0	$ 4,681.8	$ 4,471.0
Operating Segments
Segment Reporting Information
Premium Income	2,343.1	2,221.0	4,681.8	4,471.0
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,504.5	1,416.3	3,006.4	2,855.1
Operating Segments | Unum US | Group Long-term Disability
Segment Reporting Information
Premium Income	457.2	437.4	910.1	878.6
Operating Segments | Unum US | Group Short-term Disability
Segment Reporting Information
Premium Income	187.6	171.0	376.3	347.3
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	420.0	391.1	834.4	790.3
Operating Segments | Unum US | Accidental Death & Dismemberment
Segment Reporting Information
Premium Income	41.7	38.6	82.7	77.3
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	108.5	104.7	219.2	209.6
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	228.6	223.9	463.0	453.7
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	60.9	49.6	120.7	98.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	158.6	138.9	312.6	278.5
Operating Segments | Unum International | Group Long-term Disability
Segment Reporting Information
Premium Income	89.6	89.7	177.5	180.5
Operating Segments | Unum International | Group Life
Segment Reporting Information
Premium Income	28.8	27.9	56.0	56.5
Operating Segments | Unum International | Supplemental Insurance
Segment Reporting Information
Premium Income	40.2	21.3	79.1	41.5
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	420.9	395.4	840.2	793.7
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	242.4	228.6	484.6	459.9
Operating Segments | Colonial Life | Life
Segment Reporting Information
Premium Income	88.3	81.5	175.9	162.5
Operating Segments | Colonial Life | Cancer and Critical Illness
Segment Reporting Information
Premium Income	90.2	85.3	179.7	171.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	259.1	270.4	522.6	543.7
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	94.4	106.9	192.5	216.3
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	162.6	161.5	325.6	322.8
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.1	$ 2.0	$ 4.5	$ 4.6