Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2014	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 1,901.3	$ 2,008.5		$ 1,901.3
Reserves for Future Policy Contract Benefits		45,929.4	43,540.6		45,929.4
Unearned Premiums		396.6	384.2		396.6
Policy and Contract Benefits		1,529.3	1,484.6		1,529.3
Premium Income			8,082.4		7,797.2		$ 7,624.7
Net Investment Income	[1]		2,481.2		2,492.2		2,506.9
Benefits and Change in Reserves for Future Benefits			6,782.8	[2]	7,310.8	[2]	6,595.7
Amortization of Deferred Acquisition Costs			482.3		440.8		418.9
All Other Expenses	[3]		2,227.9		2,230.9		2,134.0
Reserve Increase for Long-term Care		698.2
Unclaimed Death Benefit Reserve Increase Before Taxes							95.5
Group Life
Supplementary Insurance Information, by Segment
Unclaimed Death Benefit Reserve Increase Before Taxes							49.1
Unum U.S.
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,096.5	1,136.4		1,096.5
Reserves for Future Policy Contract Benefits		11,828.0	11,273.5		11,828.0
Unearned Premiums		46.6	45.5		46.6
Policy and Contract Benefits		953.7	940.8		953.7
Premium Income			4,960.0		4,659.7		4,517.1
Net Investment Income	[1]		865.3		878.9		919.4
Benefits and Change in Reserves for Future Benefits			3,476.7	[2]	3,288.1	[2]	3,222.4
Amortization of Deferred Acquisition Costs			272.3		248.1		230.0
All Other Expenses	[3]		1,345.5		1,279.6		1,254.0
Premiums Written	[4]		3,353.8		3,144.1		3,068.0
Unclaimed Death Benefit Reserve Increase Before Taxes							75.4
Unum U.S. | Group Life
Supplementary Insurance Information, by Segment
Waiver of Premium Benefit Reserve Increase (Decrease) Before Tax							(85.0)
Unum U.K.
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		30.4	27.1		30.4
Reserves for Future Policy Contract Benefits		2,596.7	2,377.5		2,596.7
Unearned Premiums		130.6	128.2		130.6
Policy and Contract Benefits		127.6	114.3		127.6
Premium Income			576.2		607.2		556.6
Net Investment Income	[1]		124.9		151.0		148.5
Benefits and Change in Reserves for Future Benefits			394.8	[2]	431.0	[2]	413.3
Amortization of Deferred Acquisition Costs			11.3		12.5		14.7
All Other Expenses	[3]		154.4		166.9		145.2
Premiums Written	[4]		453.0		479.9		448.1
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		774.4	845.0		774.4
Reserves for Future Policy Contract Benefits		1,919.7	1,989.8		1,919.7
Unearned Premiums		30.8	32.0		30.8
Policy and Contract Benefits		191.4	191.2		191.4
Premium Income			1,338.6		1,273.7		1,232.2
Net Investment Income	[1]		145.4		145.5		144.1
Benefits and Change in Reserves for Future Benefits			683.0	[2]	660.6	[2]	667.0
Amortization of Deferred Acquisition Costs			198.7		180.2		174.2
All Other Expenses	[3]		293.3		279.5		271.8
Premiums Written	[4]		1,088.3		1,042.7		1,011.8
Colonial Life | Colonial Voluntary Life
Supplementary Insurance Information, by Segment
Unclaimed Death Benefit Reserve Increase Before Taxes							20.1
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0		0.0
Reserves for Future Policy Contract Benefits		29,585.0	27,899.8		29,585.0
Unearned Premiums		188.6	178.5		188.6
Policy and Contract Benefits		$ 256.6	238.3		256.6
Premium Income			1,207.6		1,256.6		1,318.8
Net Investment Income	[1]		1,320.0		1,281.5		1,270.2
Benefits and Change in Reserves for Future Benefits			2,228.3	[2]	2,931.1	[2]	2,293.0
Amortization of Deferred Acquisition Costs			0.0		0.0		0.0
All Other Expenses	[3]		269.5		277.0		282.6
Premiums Written	[4]		1,198.5		1,248.6		1,307.0
Closed Block | Long-term Care
Supplementary Insurance Information, by Segment
Reserve Increase for Long-term Care					698.2
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income			0.0		0.0		0.0
Net Investment Income	[1]		25.6		35.3		24.7
Benefits and Change in Reserves for Future Benefits			0.0	[2]	0.0	[2]	0.0
Amortization of Deferred Acquisition Costs			0.0		0.0		0.0
All Other Expenses	[3]		165.2		227.9		180.4
Premiums Written	[4]		$ 0.0		0.0		$ 0.0
Non-operating retirement related loss due to settlement					$ 64.4

[1] 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.

[2] Included in 2014 is a reserve charge of $698.2 million in the Closed Block segment related to our long-term care business. Included in 2013 are unclaimed death benefits reserve increases of $75.4 million in the Unum US segment and $20.1 million in the Colonial Life segment and a group life waiver of premium benefit reserve reduction of $85.0 million in the Unum US segment.

[3] Includes commissions; interest and debt expense; deferral of acquisition costs; compensation expense; non-operating retirement related loss, which in 2014 included a settlement loss of $64.4 million in the Corporate segment related to a pension plan amendment; and other expenses. Where not directly attributable to a segment, expenses are generally allocated based on activity levels, time information, and usage statistics.

[4] Excludes life insurance.