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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Premium Income	$ 1,890.7	$ 1,897.3	$ 1,905.8	$ 1,930.9	$ 1,937.2	$ 1,929.4	$ 1,927.6	$ 1,921.9	$ 7,624.7	$ 7,716.1	$ 7,514.2
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									4,517.1	4,456.5	4,296.0
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									556.6	694.6	687.6
Colonial Life
Segment Reporting Information [Line Items]
Premium Income									1,232.2	1,194.5	1,135.3
Closed Block
Segment Reporting Information [Line Items]
Premium Income									1,318.8	1,370.5	1,395.3
Group Long term Disability [Member] | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									1,553.9	1,578.8	1,580.2
Group Long term Disability [Member] | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									389.9	409.7	419.6
Group Short term Disability [Member] | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									519.6	476.7	455.2
Group Life | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									1,213.9	1,182.1	1,106.7
Group Life | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									106.4	221.3	203.6
Accidental Death Dismemberment [Member] | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									121.6	115.3	109.2
Individual Disability Recently Issued [Member] | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									465.3	477.6	464.7
Voluntary Benefits [Member] | Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									642.8	626.0	580.0
Supplemental and Voluntary Insurance [Member] | Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									60.3	63.6	64.4
Accident and Health Insurance Segment
Segment Reporting Information [Line Items]
Premium Income									5,849.6	5,868.1	5,781.1
Accident and Health Insurance Segment | Colonial Life
Segment Reporting Information [Line Items]
Premium Income									738.7	724.5	695.3
Life Insurance Product Line
Segment Reporting Information [Line Items]
Premium Income									1,775.1	1,848.0	1,733.1
Life Insurance Product Line | Colonial Life
Segment Reporting Information [Line Items]
Premium Income									221.1	209.7	190.7
Cancer and Critical Illness Colonial [Member] | Colonial Life
Segment Reporting Information [Line Items]
Premium Income									272.4	260.3	249.3
Individual Disability Insurance [Member] | Closed Block
Segment Reporting Information [Line Items]
Premium Income									687.5	736.4	787.0
Long-term Care | Closed Block
Segment Reporting Information [Line Items]
Premium Income									630.6	631.9	608.1
Other Insurance Product Line | Closed Block
Segment Reporting Information [Line Items]
Premium Income									$ 0.7	$ 2.2	$ 0.2