Selected Operating Statement Data by Segment (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting Information [Line Items]
Premium Income	$ 1,875.0	$ 1,849.8	$ 3,744.5	$ 3,713.0
Net Investment Income	637.1	629.8	1,255.8	1,242.8
Other Income	56.0	60.5	115.7	120.3
Operating Revenue	2,568.1	2,540.1	5,116.0	5,076.1
Operating Income (Loss)	341.6	337.6	656.6	673.2
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income	1,220.6	1,216.5	2,440.0	2,432.6
Net Investment Income	329.1	318.7	650.7	623.3
Other Income	29.7	30.4	60.7	60.9
Operating Revenue	1,579.4	1,565.6	3,151.4	3,116.8
Operating Income (Loss)	218.1	216.0	427.2	415.2
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income	175.8	152.9	342.9	318.7
Net Investment Income	51.1	43.1	96.0	82.9
Other Income	0	0.4	0.1	0.9
Operating Revenue	226.9	196.4	439.0	402.5
Operating Income (Loss)	54.7	52.9	103.4	113.5
Colonial Life
Segment Reporting Information [Line Items]
Premium Income	282.0	267.7	562.4	532.8
Net Investment Income	35.8	29.3	67.1	59.2
Other Income	0.2	0.1	0.3	0.3
Operating Revenue	318.0	297.1	629.8	592.3
Operating Income (Loss)	75.2	73.9	144.2	146.9
Individual Disability-Closed Block
Segment Reporting Information [Line Items]
Premium Income	196.3	212.5	398.6	426.5
Net Investment Income	180.3	187.3	362.3	378.0
Other Income	21.7	23.1	44.3	44.6
Operating Revenue	398.3	422.9	805.2	849.1
Operating Income (Loss)	10.4	12.4	20.3	24.1
Corporate and Other
Segment Reporting Information [Line Items]
Premium Income	0.3	0.2	0.6	2.4
Net Investment Income	40.8	51.4	79.7	99.4
Other Income	4.4	6.5	10.3	13.6
Operating Revenue	45.5	58.1	90.6	115.4
Operating Income (Loss)	$ (16.8)	$ (17.6)	$ (38.5)	$ (26.5)