Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,324.0	$ 2,309.4
Reserves for Future Policy Contract Benefits		47,780.1	44,841.9
Unearned Premiums		363.9	363.3
Policy and Contract Benefits		1,745.5	1,695.7
Premium Income		9,365.6	8,986.1	$ 8,597.1
Net Investment Income	[1]	2,435.3	2,453.7	2,451.7
Benefits and Change in Reserves for Future Benefits	[2]	7,496.2	8,020.4	7,055.7
Amortization of Deferred Acquisition Costs		609.9	565.5	527.1
All Other Expenses	[3]	2,510.7	2,384.8	2,300.0
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,223.0	1,239.4
Reserves for Future Policy Contract Benefits		11,367.8	10,840.9
Unearned Premiums		46.0	47.2
Policy and Contract Benefits		1,117.5	1,091.5
Premium Income		6,016.6	5,736.4	5,443.5
Net Investment Income		739.4	778.7	811.2
Benefits and Change in Reserves for Future Benefits		4,022.1	3,856.5	3,693.4
Amortization of Deferred Acquisition Costs		344.0	315.1	293.6
All Other Expenses		1,501.6	1,447.4	1,398.0
Premiums Written		4,073.9	3,873.0	3,701.3
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		26.4	20.0
Reserves for Future Policy Contract Benefits		2,566.6	2,340.5
Unearned Premiums		129.9	123.3
Policy and Contract Benefits		157.3	115.6
Premium Income		630.5	568.8	513.0
Net Investment Income		122.5	117.2	120.2
Benefits and Change in Reserves for Future Benefits		469.8	419.8	381.9
Amortization of Deferred Acquisition Costs		7.1	8.2	9.1
All Other Expenses		168.8	144.5	131.2
Premiums Written		443.7	455.5	404.2
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,074.6	1,050.0
Reserves for Future Policy Contract Benefits		2,477.2	2,330.2
Unearned Premiums		36.8	36.3
Policy and Contract Benefits		189.4	206.2
Premium Income		1,685.0	1,603.8	1,511.4
Net Investment Income		148.0	151.2	144.9
Benefits and Change in Reserves for Future Benefits	[2]	865.0	824.9	788.6
Amortization of Deferred Acquisition Costs		258.8	242.2	224.4
All Other Expenses		368.1	353.9	331.8
Premiums Written		1,249.6	1,277.3	1,210.6
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0	0.0
Reserves for Future Policy Contract Benefits		31,368.5	29,330.3
Unearned Premiums		151.2	156.5
Policy and Contract Benefits		281.3	282.4
Premium Income		1,033.5	1,077.1	1,129.2
Net Investment Income		1,404.9	1,377.1	1,354.0
Benefits and Change in Reserves for Future Benefits		2,139.3	2,919.2	2,191.8
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses		232.7	235.7	247.3
Premiums Written		1,020.8	1,065.7	1,109.8
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0	0.0	0.0
Net Investment Income		20.5	29.5	21.4
Benefits and Change in Reserves for Future Benefits		0.0	0.0	0.0
Amortization of Deferred Acquisition Costs		0.0	0.0	0.0
All Other Expenses		239.5	203.3	191.7
Premiums Written		$ 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

[3] Includes commissions, interest and debt expense, cost related to early retirement of debt, 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