Selected Operating Statement Data by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Premium Income	$ 1,937.2	$ 1,929.4	$ 1,927.6	$ 1,921.9	$ 1,888.5	$ 1,881.2	$ 1,875.0	$ 1,869.5	$ 7,716.1	$ 7,514.2	$ 7,431.4
Net Investment Income	643.0	619.2	633.5	619.5	634.6	629.2	637.1	618.7	2,515.2	2,519.6	2,495.5
Other Income									227.9	249.1	241.6
Operating Revenue									10,459.2	10,282.9	10,168.5
Operating Income (Loss)									1,239.7	370.1	1,327.4
Interest and Debt Expense									145.4	143.3	141.8
Depreciation and Amortization									468.5	455.0	458.1
Unum U.S.
Segment Reporting Information [Line Items]
Premium Income									4,456.5	4,296.0	4,255.4
Net Investment Income									952.3	951.4	941.5
Other Income									124.6	121.6	122.8
Operating Revenue									5,533.4	5,369.0	5,319.7
Operating Income (Loss)									847.1	816.9	769.0
Interest and Debt Expense									1.1	1.0	1.2
Depreciation and Amortization									255.6	245.9	245.6
Unum U.K.
Segment Reporting Information [Line Items]
Premium Income									694.6	687.6	650.6
Net Investment Income									170.8	189.9	170.5
Other Income									0.1	0.3	1.2
Operating Revenue									865.5	877.8	822.3
Operating Income (Loss)									131.3	190.7	209.0
Interest and Debt Expense									0	0	0
Depreciation and Amortization									27.2	26.8	24.8
Colonial Life
Segment Reporting Information [Line Items]
Premium Income									1,194.5	1,135.3	1,075.7
Net Investment Income									138.6	132.4	122.5
Other Income									0.3	0.5	0.7
Operating Revenue									1,333.4	1,268.2	1,198.9
Operating Income (Loss)									274.3	270.1	269.7
Interest and Debt Expense									0	0	0
Depreciation and Amortization									181.0	164.6	163.0
Closed Block
Segment Reporting Information [Line Items]
Premium Income									1,370.5	1,395.3	1,449.7
Net Investment Income									1,230.5	1,189.7	1,166.4
Other Income									100.1	106.1	113.6
Operating Revenue									2,701.1	2,691.1	2,729.7
Operating Income (Loss)									95.5	(829.2)	118.6
Interest and Debt Expense									10.4	10.5	11.7
Depreciation and Amortization									3.9	16.9	23.6
Corporate
Segment Reporting Information [Line Items]
Premium Income									0	0	0
Net Investment Income									23.0	56.2	94.6
Other Income									2.8	20.6	3.3
Operating Revenue									25.8	76.8	97.9
Operating Income (Loss)									(108.5)	(78.4)	(38.9)
Interest and Debt Expense									133.9	131.8	128.9
Depreciation and Amortization									$ 0.8	$ 0.8	$ 1.1