Selected Operating Statement Data by 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
Net Investment Income	629.4	615.5	626.1	621.1	643.0	619.2	633.5	619.5	2,492.1	2,515.2	2,519.6
Other Income									230.2	227.9	249.1
Operating Revenue									10,347.0	10,459.2	10,282.9
Operating Income (Loss)									1,241.8	1,239.7	1,323.2
Interest and Debt Expense									149.4	145.4	143.3
Depreciation, Depletion and Amortization									509.8	468.5	455.0
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									4,517.1	4,456.5	4,296.0
Net Investment Income									929.6	952.3	951.4
Other Income									128.3	124.6	121.6
Operating Revenue									5,575.0	5,533.4	5,369.0
Operating Income (Loss)									859.0	847.1	816.9
Interest and Debt Expense									0.1	1.1	1.0
Depreciation, Depletion and Amortization									292.5	255.6	245.9
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									556.6	694.6	687.6
Net Investment Income									148.5	170.8	189.9
Other Income									0.1	0.1	0.3
Operating Revenue									705.2	865.5	877.8
Operating Income (Loss)									132.0	131.3	190.7
Interest and Debt Expense									0	0	0
Depreciation, Depletion and Amortization									22.5	27.2	26.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income									1,232.2	1,194.5	1,135.3
Net Investment Income									145.4	138.6	132.4
Other Income									0.2	0.3	0.5
Operating Revenue									1,377.8	1,333.4	1,268.2
Operating Income (Loss)									284.9	274.3	270.1
Interest and Debt Expense									0	0	0
Depreciation, Depletion and Amortization									188.7	181.0	164.6
Closed Block
Segment Reporting Information [Line Items]
Premium Income									1,318.8	1,370.5	1,395.3
Net Investment Income									1,272.3	1,230.5	1,189.7
Other Income									93.9	100.1	106.1
Operating Revenue									2,685.0	2,701.1	2,691.1
Operating Income (Loss)									109.4	95.5	123.9
Interest and Debt Expense									8.4	10.4	10.5
Depreciation, Depletion and Amortization									5.2	3.9	16.9
Corporate
Segment Reporting Information [Line Items]
Premium Income									0	0	0
Net Investment Income									(3.7)	23.0	56.2
Other Income									7.7	2.8	20.6
Operating Revenue									4.0	25.8	76.8
Operating Income (Loss)									(143.5)	(108.5)	(78.4)
Interest and Debt Expense									140.9	133.9	131.8
Depreciation, Depletion and Amortization									$ 0.9	$ 0.8	$ 0.8