Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Premium Income	$ 1,905.8	$ 1,927.6	$ 3,836.7	$ 3,849.5
Net Investment Income	626.1	633.5	1,247.2	1,253.0
Other Income	56.7	58.9	119.2	116.4
Operating Revenue	2,588.6	2,620.0	5,203.1	5,218.9
Operating Income (Loss)	311.1	310.1	619.5	619.3
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,131.5	1,115.0	2,271.2	2,227.0
Net Investment Income	231.1	236.9	466.8	475.5
Other Income	32.1	30.7	68.8	61.8
Operating Revenue	1,394.7	1,382.6	2,806.8	2,764.3
Operating Income (Loss)	214.0	212.7	422.1	418.6
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	137.6	173.2	281.4	343.9
Net Investment Income	45.4	49.2	76.2	85.7
Other Income	(0.1)	0	0	0.1
Operating Revenue	182.9	222.4	357.6	429.7
Operating Income (Loss)	33.5	30.0	64.8	68.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	307.9	296.9	615.0	593.2
Net Investment Income	34.3	34.0	74.0	69.6
Other Income	0.1	0.1	0.1	0.2
Operating Revenue	342.3	331.0	689.1	663.0
Operating Income (Loss)	71.1	67.6	146.5	137.3
Closed Block
Segment Reporting Information [Line Items]
Premium Income	328.8	342.5	669.1	685.4
Net Investment Income	316.1	304.5	629.2	606.1
Other Income	23.8	26.6	47.9	52.5
Operating Revenue	668.7	673.6	1,346.2	1,344.0
Operating Income (Loss)	29.6	25.7	56.9	41.1
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0	0	0
Net Investment Income	(0.8)	8.9	1.0	16.1
Other Income	0.8	1.5	2.4	1.8
Operating Revenue	0	10.4	3.4	17.9
Operating Income (Loss)	$ (37.1)	$ (25.9)	$ (70.8)	$ (46.5)