Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,272.6		$ 2,324.0
Reserves for Future Policy Contract Benefits		49,653.0		47,780.1
Unearned Premiums		349.3		363.9
Policy and Contract Benefits		1,855.4		1,745.5
Premium Income		9,378.1		9,365.6	$ 8,986.1
Net Investment Income	[1]	2,360.7		2,435.3	2,453.7
Benefits and Change in Reserves for Future Benefits		8,972.9	[2]	7,496.2	8,020.4	[2]
Amortization of Deferred Acquisition Costs		606.1		609.9	565.5
All Other Expenses	[3]	2,619.1		2,510.7	2,384.8
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,168.7		1,223.0
Reserves for Future Policy Contract Benefits		11,681.4		11,367.8
Unearned Premiums		44.0		46.0
Policy and Contract Benefits		1,191.2		1,117.5
Premium Income		6,018.9		6,016.6	5,736.4
Net Investment Income		720.3		739.4	778.7
Benefits and Change in Reserves for Future Benefits		4,138.7		4,022.1	3,856.5
Amortization of Deferred Acquisition Costs		341.0		344.0	315.1
All Other Expenses		1,589.0		1,501.6	1,447.4
Premiums Written	[4]	4,088.6		4,073.9	3,873.0
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		32.0		26.4
Reserves for Future Policy Contract Benefits		2,794.2		2,566.6
Unearned Premiums		123.3		129.9
Policy and Contract Benefits		175.6		157.3
Premium Income		652.8		630.5	568.8
Net Investment Income		104.6		122.5	117.2
Benefits and Change in Reserves for Future Benefits		500.9		469.8	419.8
Amortization of Deferred Acquisition Costs		7.4		7.1	8.2
All Other Expenses		173.0		168.8	144.5
Premiums Written	[4]	456.0		443.7	455.5
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,071.9		1,074.6
Reserves for Future Policy Contract Benefits		2,628.5		2,477.2
Unearned Premiums		36.9		36.8
Policy and Contract Benefits		217.2		189.4
Premium Income		1,712.0		1,685.0	1,603.8
Net Investment Income		155.7		148.0	151.2
Benefits and Change in Reserves for Future Benefits		906.5		865.0	824.9
Amortization of Deferred Acquisition Costs		257.7		258.8	242.2
All Other Expenses		369.2		368.1	353.9
Premiums Written	[4]	1,252.4		1,249.6	1,277.3
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0		0.0
Reserves for Future Policy Contract Benefits		32,548.9		31,368.5
Unearned Premiums		145.1		151.2
Policy and Contract Benefits		271.4		281.3
Premium Income		994.4		1,033.5	1,077.1
Net Investment Income		1,370.3		1,404.9	1,377.1
Benefits and Change in Reserves for Future Benefits		3,426.8	[2]	2,139.3	2,919.2	[2]
Amortization of Deferred Acquisition Costs		0.0		0.0	0.0
All Other Expenses		240.2	[3]	232.7	235.7
Premiums Written	[4]	979.3		1,020.8	1,065.7
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0		0.0	0.0
Net Investment Income		9.8		20.5	29.5
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		247.7	[3]	239.5	203.3
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 2020 and 2018 are reserve increases of $151.5 million and $750.8 million, respectively in the Closed Block segment related to our long-term care business, as well as a reserve increase of $17.5 million in 2020 related to our group pension Closed Block business. Also included in 2020 is an increase in benefits and change in reserves for future benefits of $1,284.5 million in 2020 resulting from the recognition of the adjustment related to unrealized investment gains and losses previously recognized in accumulated other comprehensive income related to the Closed Block individual disability reinsurance transaction.

[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 2020 is the amortization of the cost of reinsurance of $2.6 million and transaction costs of $21.0 million related to the Closed Block individual disability reinsurance transaction. Also included in 2020 is a ROU asset impairment of $12.7 million related to one of our operating leases for office space that we do not plan to continue using to support general operations and costs related to organizational design updates of $23.3 million in Corporate.

[4] Excludes life insurance.