Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Premium Income	$ 1,888.5	$ 1,881.2	$ 1,875.0	$ 1,869.5	$ 1,868.2	$ 1,850.2	$ 1,849.8	$ 1,863.2	$ 7,514.2	$ 7,431.4	$ 7,475.5
Net Investment Income	634.6	629.2	637.1	618.7	634.3	618.4	629.8	613.0	2,519.6	2,495.5	2,346.6
Other Income									249.1	241.6	257.2
Operating Revenue									10,282.9	10,168.5	10,079.3
Operating Income (Loss)									262.1	1,306.6	1,280.6
Interest and Debt Expense									143.3	141.8	125.4
Depreciation and Amortization									623.1	631.9	610.3
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									4,296.0	4,255.4	4,278.4
Net Investment Income									951.4	941.5	934.3
Other Income									121.6	122.8	118.8
Operating Revenue									5,369.0	5,319.7	5,331.5
Operating Income (Loss)									819.8	769.1	717.6
Interest and Debt Expense									1.0	1.2	2.0
Depreciation and Amortization									356.5	360.9	344.7
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									687.6	650.6	686.1
Net Investment Income									189.9	170.5	124.5
Other Income									0.3	1.2	2.4
Operating Revenue									877.8	822.3	813.0
Operating Income (Loss)									192.0	208.8	249.6
Interest and Debt Expense									0	0	0
Depreciation and Amortization									40.7	38.2	42.4
Colonial Life
Segment Reporting Information [Line Items]
Premium Income									1,135.3	1,075.7	1,015.1
Net Investment Income									132.4	122.5	114.3
Other Income									0.5	0.7	0.5
Operating Revenue									1,268.2	1,198.9	1,129.9
Operating Income (Loss)									282.1	282.2	280.9
Interest and Debt Expense									0	0	0
Depreciation and Amortization									202.4	200.1	192.0
Closed Block
Segment Reporting Information [Line Items]
Premium Income									1,395.3	1,449.7	1,495.9
Net Investment Income									1,189.7	1,166.4	1,106.8
Other Income									106.1	113.6	131.1
Operating Revenue									2,691.1	2,729.7	2,733.8
Operating Income (Loss)									(921.5)	117.6	124.4
Interest and Debt Expense									10.5	11.7	16.6
Depreciation and Amortization									22.7	31.6	29.6
Corporate
Segment Reporting Information [Line Items]
Premium Income									0	0	0
Net Investment Income									56.2	94.6	66.7
Other Income									20.6	3.3	4.4
Operating Revenue									76.8	97.9	71.1
Operating Income (Loss)									(110.3)	(71.1)	(91.9)
Interest and Debt Expense									131.8	128.9	106.8
Depreciation and Amortization									$ 0.8	$ 1.1	$ 1.6