Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Premium Income	$ 1,927.6	$ 1,875.0	$ 3,849.5	$ 3,744.5
Net Investment Income	633.5	637.1	1,253.0	1,255.8
Other Income	58.9	56.0	116.4	115.7
Operating Revenue	2,620.0	2,568.1	5,218.9	5,116.0
Operating Income (Loss)	310.1	346.1	619.3	666.3
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,115.0	1,069.8	2,227.0	2,138.6
Net Investment Income	236.9	238.9	475.5	472.6
Other Income	30.7	29.5	61.8	60.5
Operating Revenue	1,382.6	1,338.2	2,764.3	2,671.7
Operating Income (Loss)	212.7	205.6	418.6	400.3
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	173.2	175.8	343.9	342.9
Net Investment Income	49.2	51.1	85.7	96.0
Other Income	0	0	0.1	0.1
Operating Revenue	222.4	226.9	429.7	439.0
Operating Income (Loss)	30.0	55.2	68.8	104.1
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	296.9	282.0	593.2	562.4
Net Investment Income	34.0	35.8	69.6	67.1
Other Income	0.1	0.2	0.2	0.3
Operating Revenue	331.0	318.0	663.0	629.8
Operating Income (Loss)	67.6	71.9	137.3	138.4
Closed Block
Segment Reporting Information [Line Items]
Premium Income	342.5	347.4	685.4	700.6
Net Investment Income	304.5	294.5	606.1	588.8
Other Income	26.6	26.7	52.5	54.2
Operating Revenue	673.6	668.6	1,344.0	1,343.6
Operating Income (Loss)	25.7	30.4	41.1	62.3
Corporate
Segment Reporting Information [Line Items]
Premium Income	0	0	0	0
Net Investment Income	8.9	16.8	16.1	31.3
Other Income	1.5	(0.4)	1.8	0.6
Operating Revenue	10.4	16.4	17.9	31.9
Operating Income (Loss)	$ (25.9)	$ (17.0)	$ (46.5)	$ (38.8)