Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,094.2	$ 2,008.5
Reserves for Future Policy Contract Benefits		44,245.9	43,540.6
Unearned Premiums		363.7	384.2
Policy and Contract Benefits		1,507.9	1,484.6
Premium Income		8,357.7	8,082.4	$ 7,797.2
Net Investment Income	[1]	2,459.0	2,481.2	2,492.2
Benefits and Change in Reserves for Future Benefits	[2]	6,941.8	6,782.8	7,310.8
Amortization of Deferred Acquisition Costs		493.0	482.3	440.8
All Other Expenses	[3]	2,264.0	2,227.9	2,230.9
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,176.5	1,136.4
Reserves for Future Policy Contract Benefits		11,312.6	11,273.5
Unearned Premiums		46.9	45.5
Policy and Contract Benefits		977.2	940.8
Premium Income		5,240.9	4,960.0	4,659.7
Net Investment Income	[1]	828.7	865.3	878.9
Benefits and Change in Reserves for Future Benefits	[2]	3,624.3	3,476.7	3,288.1
Amortization of Deferred Acquisition Costs		275.2	272.3	248.1
All Other Expenses	[3]	1,369.2	1,345.5	1,279.6
Premiums Written	[4]	3,564.4	3,353.8	3,144.1
Unum UK
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		21.4	27.1
Reserves for Future Policy Contract Benefits		2,127.9	2,377.5
Unearned Premiums		110.3	128.2
Policy and Contract Benefits		102.4	114.3
Premium Income		529.3	576.2	607.2
Net Investment Income	[1]	118.1	124.9	151.0
Benefits and Change in Reserves for Future Benefits	[2]	367.4	394.8	431.0
Amortization of Deferred Acquisition Costs		9.7	11.3	12.5
All Other Expenses	[3]	141.9	154.4	166.9
Premiums Written	[4]	435.4	453.0	479.9
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		896.3	845.0
Reserves for Future Policy Contract Benefits		2,103.5	1,989.8
Unearned Premiums		33.5	32.0
Policy and Contract Benefits		195.8	191.2
Premium Income		1,417.1	1,338.6	1,273.7
Net Investment Income	[1]	141.5	145.4	145.5
Benefits and Change in Reserves for Future Benefits	[2]	726.4	683.0	660.6
Amortization of Deferred Acquisition Costs		208.1	198.7	180.2
All Other Expenses	[3]	311.1	293.3	279.5
Premiums Written	[4]	1,145.0	1,088.3	1,042.7
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		28,701.9	27,899.8
Unearned Premiums		173.0	178.5
Policy and Contract Benefits		232.5	238.3
Premium Income		1,170.4	1,207.6	1,256.6
Net Investment Income	[1]	1,352.2	1,320.0	1,281.5
Benefits and Change in Reserves for Future Benefits	[2]	2,223.7	2,228.3	2,931.1
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	255.4	269.5	277.0
Premiums Written	[4]	1,161.8	1,198.5	1,248.6
Closed Block | Long-term Care
Supplementary Insurance Information, by Segment
Reserve Increase for Long-term Care, Before Tax			698.2
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	18.5	25.6	35.3
Benefits and Change in Reserves for Future Benefits	[2]	0.0	0.0	0.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	186.4	165.2	227.9
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.

[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.