Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,309.4	$ 2,184.6
Reserves for Future Policy Contract Benefits		44,841.9	45,601.6
Unearned Premiums		363.3	373.1
Policy and Contract Benefits		1,695.7	1,605.2
Premium Income		8,986.1	8,597.1	$ 8,357.7
Net Investment Income	[1]	2,453.7	2,451.7	2,459.0
Benefits and Change in Reserves for Future Benefits	[2]	8,020.4	7,055.7	6,941.8
Amortization of Deferred Acquisition Costs		565.5	527.1	493.0
All Other Expenses	[3]	2,384.8	2,300.0	2,264.0
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,239.4	1,205.4
Reserves for Future Policy Contract Benefits		10,840.9	11,382.1
Unearned Premiums		47.2	49.0
Policy and Contract Benefits		1,091.5	1,059.8
Premium Income		5,736.4	5,443.5	5,240.9
Net Investment Income	[1]	778.7	811.2	828.7
Benefits and Change in Reserves for Future Benefits	[2]	3,856.5	3,693.4	3,624.3
Amortization of Deferred Acquisition Costs		315.1	293.6	275.2
All Other Expenses	[3]	1,447.4	1,398.0	1,369.2
Premiums Written	[4]	3,873.0	3,701.3	3,564.4
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		20.0	21.3
Reserves for Future Policy Contract Benefits		2,340.5	2,361.6
Unearned Premiums		123.3	125.7
Policy and Contract Benefits		115.6	108.5
Premium Income		568.8	513.0	529.3
Net Investment Income	[1]	117.2	120.2	118.1
Benefits and Change in Reserves for Future Benefits	[2]	419.8	381.9	367.4
Amortization of Deferred Acquisition Costs		8.2	9.1	9.7
All Other Expenses	[3]	144.5	131.2	141.9
Premiums Written	[4]	455.5	404.2	435.4
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,050.0	957.9
Reserves for Future Policy Contract Benefits		2,330.2	2,224.2
Unearned Premiums		36.3	34.7
Policy and Contract Benefits		206.2	212.1
Premium Income		1,603.8	1,511.4	1,417.1
Net Investment Income	[1]	151.2	144.9	141.5
Benefits and Change in Reserves for Future Benefits	[2]	824.9	788.6	726.4
Amortization of Deferred Acquisition Costs		242.2	224.4	208.1
All Other Expenses	[3]	353.9	331.8	311.1
Premiums Written	[4]	1,277.3	1,210.6	1,145.0
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		29,330.3	29,633.7
Unearned Premiums		156.5	163.7
Policy and Contract Benefits		282.4	224.8
Premium Income		1,077.1	1,129.2	1,170.4
Net Investment Income	[1]	1,377.1	1,354.0	1,352.2
Benefits and Change in Reserves for Future Benefits	[2]	2,919.2	2,191.8	2,223.7
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses	[3]	235.7	247.3	255.4
Premiums Written	[4]	1,065.7	1,109.8	1,161.8
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income	[1]	29.5	21.4	18.5
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]	203.3	191.7	186.4
Premiums Written	[4]	$ 0.0	$ 0.0	$ 0.0

[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 2018 is a reserve charge of $750.8 million in the Closed Block segment related to our long-term care business. Included in 2017 are unclaimed death benefits reserve increases of $26.6 million in Unum US and $12.4 million in Colonial Life.

[3] Includes commissions; interest and debt expense; deferral of acquisition costs; compensation expense; and other expenses. Where not directly attributable to a segment, expenses are generally allocated based on activity levels, time information, and usage statistics. Included in 2017 is a loss contingency accrual for a guaranty fund assessment of $20.6 million in Corporate.

[4] Excludes life insurance.