Schedule III Supplementary Insurance Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended																																						3 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Unum U.S.		Dec. 31, 2011 Unum U.S.		Dec. 31, 2010 Unum U.S.		Dec. 31, 2012 Unum U.K.		Dec. 31, 2011 Unum U.K.		Dec. 31, 2010 Unum U.K.		Dec. 31, 2012 Colonial Life		Dec. 31, 2011 Colonial Life		Dec. 31, 2010 Colonial Life		Dec. 31, 2012 Closed Block		Dec. 31, 2011 Closed Block		Dec. 31, 2010 Closed Block		Dec. 31, 2012 Corporate		Dec. 31, 2011 Corporate		Dec. 31, 2010 Corporate		Dec. 31, 2011 Claim Reserve Total [Member] Long-term Care	Dec. 31, 2011 Claim Reserve Total [Member] Individual Disability Closed Block	Dec. 31, 2011 Before Tax [Member] Long-term Care	Dec. 31, 2011 Before Tax [Member] Individual Disability Closed Block	Dec. 31, 2011 Adjustments for New Accounting Pronouncement [Member]	Dec. 31, 2010 Adjustments for New Accounting Pronouncement [Member]	Dec. 31, 2011 Adjustments for New Accounting Pronouncement [Member] Before Tax [Member]	Dec. 31, 2011 Adjustments for New Accounting Pronouncement [Member] Before Tax [Member]	Dec. 31, 2010 Adjustments for New Accounting Pronouncement [Member] Before Tax [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	$ 1,755.5		$ 1,677.1	[1]			$ 1,024.3		$ 971.8	[1]			$ 38.8		$ 40.9	[1]			$ 692.4		$ 664.4	[1]			$ 0		$ 0	[1]
Reserves for Future Policy Contract Benefits	44,694.4		43,051.9				12,449.3		12,280.2				2,487.1		2,172.5				1,691.3		1,598.1				28,066.7		27,001.1
Unearned Premiums	426.7		433.2				46.1		43.4				142.9		143.2				29.9		28.5				207.8		218.1
Policy and Contract Benefits	1,484.6		1,494.0				890.3		889.2				155.7		135.6				149.8		134.7				288.8		334.5
Premium Income	7,716.1		7,514.2		7,431.4		4,456.5		4,296.0		4,255.4		694.6		687.6		650.6		1,194.5		1,135.3		1,075.7		1,370.5		1,395.3		1,449.7		0		0		0
Net Investment Income	2,515.2	[2]	2,519.6	[2]	2,495.5	[2]	952.3	[2]	951.4	[2]	941.5	[2]	170.8	[2]	189.9	[2]	170.5	[2]	138.6	[2]	132.4	[2]	122.5	[2]	1,230.5	[2]	1,189.7	[2]	1,166.4	[2]	23.0	[2]	56.2	[2]	94.6	[2]
Benefits and Change in Reserves for Future Benefits	6,722.2		7,209.5		6,354.1		3,238.6		3,113.5		3,124.4		541.4		493.8		435.8		627.3		589.4		534.7		2,314.9		3,012.8	[3]	2,259.2		0		0		0
Amortization of Deferred Acquisition Costs	378.7		365.7	[1]	373.3	[1]	196.5		188.1	[1]	192.6	[1]	15.7		15.3	[1]	13.6	[1]	166.5		151.2	[1]	150.1	[1]	0		11.1	[1]	17.0	[1]	0		0	[1]	0	[1]
All Other Expenses	2,165.0	[4]	2,369.5	[1],[4]	2,145.9	[1],[4]	1,251.2	[4]	1,250.5	[1],[4]	1,233.7	[1],[4]	177.1	[4]	178.0	[1],[4]	163.9	[1],[4]	265.3	[4]	257.5	[1],[4]	244.4	[1],[4]	290.7	[4]	496.4	[1],[4]	334.9	[1],[4]	180.7	[4]	187.1	[1],[4]	169.0	[1],[4]
Premiums Written							3,045.0	[5]	2,965.8	[5]	2,938.9	[5]	466.3	[5]	480.0	[5]	469.4	[5]	986.3	[5]	944.9	[5]	901.9	[5]	1,358.6	[5]	1,385.1	[5]	1,430.4	[5]	0	[5]	0	[5]	0	[5]
Reserve Increase Due to Unlocking																																					248.1	183.5	573.6	183.5
Impairment of Deferred Acquisition Costs	$ 0		$ 196.0		$ 0																																$ 573.6				$ 196.0	$ 0	$ 196.0	$ 196.0	$ 0

[1] Prior period results have been adjusted for the retrospective adoption of the accounting standards update related to deferred acquisition costs. See Note 1 of the "Notes to Consolidated Financial Statements" contained herein in Item 8 for further discussion.

[2] Net investment income is allocated based upon segmentation. Each segment has its own specifically identified assets and receives the investment income generated by those assets.

[3] Included in 2011 in the Closed Block segment is a reserve charge of $573.6 million related to our long-term care business and a reserve charge of $183.5 million related to our individual disability business.

[4] Includes commissions, interest and debt expense, deferral of acquisition costs, compensation expense, non-operating retirement related loss, and other expenses. Included in 2011 in the Closed Block segment is an impairment charge of $196.0 million related to long-term care deferred policy acquisition costs. Where not directly attributable to a segment, expenses are generally allocated based on activity levels, time information, and usage statistics.

[5] Excludes life insurance.