Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information
Premium Income	$ 2,037.8	$ 2,020.8	$ 2,017.5	$ 2,006.3	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 8,082.4	$ 7,797.2	$ 7,624.7
Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income									6,099.2	5,926.0	5,849.6
Life Insurance
Segment Reporting Information
Premium Income									1,983.2	1,871.2	1,775.1
Operating Segments
Segment Reporting Information
Premium Income									8,082.4	7,797.2	7,624.7
Operating Segments | Unum U.S.
Segment Reporting Information
Premium Income									4,960.0	4,659.7	4,517.1
Operating Segments | Unum U.S. | Group Long term Disability
Segment Reporting Information
Premium Income									1,644.7	1,553.5	1,553.9
Operating Segments | Unum U.S. | Group Short term Disability
Segment Reporting Information
Premium Income									607.4	558.1	519.6
Operating Segments | Unum U.S. | Group Life
Segment Reporting Information
Premium Income									1,347.4	1,262.3	1,213.9
Operating Segments | Unum U.S. | Accidental Death Dismemberment
Segment Reporting Information
Premium Income									131.7	125.9	121.6
Operating Segments | Unum U.S. | Individual Disability Insurance
Segment Reporting Information
Premium Income									478.9	466.1	465.3
Operating Segments | Unum U.S. | Voluntary Benefits
Segment Reporting Information
Premium Income									749.9	693.8	642.8
Operating Segments | Unum U.K.
Segment Reporting Information
Premium Income									576.2	607.2	556.6
Operating Segments | Unum U.K. | Group Long term Disability
Segment Reporting Information
Premium Income									397.4	418.9	389.9
Operating Segments | Unum U.K. | Group Life
Segment Reporting Information
Premium Income									121.5	133.2	106.4
Operating Segments | Unum U.K. | Supplemental Insurance
Segment Reporting Information
Premium Income									57.3	55.1	60.3
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,338.6	1,273.7	1,232.2
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income									789.0	759.8	738.7
Operating Segments | Colonial Life | Life Insurance
Segment Reporting Information
Premium Income									252.4	231.8	221.1
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									297.2	282.1	272.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,207.6	1,256.6	1,318.8
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income									572.4	624.8	687.5
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									633.5	630.9	630.6
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 1.7	$ 0.9	$ 0.7