Premium Income by Major Line of Business within Each Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information
Premium Income	$ 1,967.9	$ 1,947.2	$ 1,943.6	$ 1,938.5	$ 1,890.7	$ 1,897.3	$ 1,905.8	$ 1,930.9	$ 7,797.2	$ 7,624.7	$ 7,716.1
Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income									5,926.0	5,849.6	5,868.1
Life Insurance Product Line
Segment Reporting Information
Premium Income									1,871.2	1,775.1	1,848.0
Operating Segments
Segment Reporting Information
Premium Income									7,797.2	7,624.7	7,716.1
Operating Segments | Unum U.S.
Segment Reporting Information
Premium Income									4,659.7	4,517.1	4,456.5
Operating Segments | Unum U.S. | Group Long term Disability
Segment Reporting Information
Premium Income									1,553.5	1,553.9	1,578.8
Operating Segments | Unum U.S. | Group Short term Disability
Segment Reporting Information
Premium Income									558.1	519.6	476.7
Operating Segments | Unum U.S. | Group Life
Segment Reporting Information
Premium Income									1,262.3	1,213.9	1,182.1
Operating Segments | Unum U.S. | Accidental Death Dismemberment
Segment Reporting Information
Premium Income									125.9	121.6	115.3
Operating Segments | Unum U.S. | Individual Disability Insurance
Segment Reporting Information
Premium Income									466.1	465.3	477.6
Operating Segments | Unum U.S. | Voluntary Benefits
Segment Reporting Information
Premium Income									693.8	642.8	626.0
Operating Segments | Unum U.K.
Segment Reporting Information
Premium Income									607.2	556.6	694.6
Operating Segments | Unum U.K. | Group Long term Disability
Segment Reporting Information
Premium Income									418.9	389.9	409.7
Operating Segments | Unum U.K. | Group Life
Segment Reporting Information
Premium Income									133.2	106.4	221.3
Operating Segments | Unum U.K. | Supplemental Insurance
Segment Reporting Information
Premium Income									55.1	60.3	63.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income									1,273.7	1,232.2	1,194.5
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income									759.8	738.7	724.5
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income									231.8	221.1	209.7
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income									282.1	272.4	260.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income									1,256.6	1,318.8	1,370.5
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income									624.8	687.5	736.4
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income									630.9	630.6	631.9
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income									$ 0.9	$ 0.7	$ 2.2