Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,207.9		$ 2,272.6
Reserves for Future Policy Contract Benefits		48,007.5		49,653.0
Unearned Premiums		347.5		349.3
Policy and Contract Benefits		1,907.7		1,855.4
Premium Income		9,481.0		9,378.1	$ 9,365.6
Net Investment Income	[1]	2,213.2		2,360.7	2,435.3
Benefits and Change in Reserves for Future Benefits		7,598.6	[2]	8,972.9	7,496.2	[2]
Amortization of Deferred Acquisition Costs		586.1		606.1	609.9
All Other Expenses	[3]	2,766.1		2,619.1	2,510.7
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,096.2		1,168.7
Reserves for Future Policy Contract Benefits		10,959.4		11,681.4
Unearned Premiums		41.5		44.0
Policy and Contract Benefits		1,262.5		1,191.2
Premium Income		6,078.0		6,018.9	6,016.6
Net Investment Income		721.6		720.3	739.4
Benefits and Change in Reserves for Future Benefits		4,338.8		4,138.7	4,022.1
Amortization of Deferred Acquisition Costs		319.0		341.0	344.0
All Other Expenses		1,631.9		1,589.0	1,501.6
Premiums Written	[4]	4,168.9		4,088.6	4,073.9
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		35.4		32.0
Reserves for Future Policy Contract Benefits		2,765.5		2,794.2
Unearned Premiums		131.8		123.3
Policy and Contract Benefits		171.2		175.6
Premium Income		717.0		652.8	630.5
Net Investment Income		132.7	[1]	104.6	122.5
Benefits and Change in Reserves for Future Benefits		556.2		500.9	469.8
Amortization of Deferred Acquisition Costs		8.0		7.4	7.1
All Other Expenses		180.4		173.0	168.8
Premiums Written	[4]	520.6		456.0	443.7
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,076.3		1,071.9
Reserves for Future Policy Contract Benefits		2,730.1		2,628.5
Unearned Premiums		36.3		36.9
Policy and Contract Benefits		210.3		217.2
Premium Income		1,690.2		1,712.0	1,685.0
Net Investment Income		172.0		155.7	148.0
Benefits and Change in Reserves for Future Benefits		910.4		906.5	865.0
Amortization of Deferred Acquisition Costs		259.1		257.7	258.8
All Other Expenses		364.5		369.2	368.1
Premiums Written	[4]	1,223.3		1,252.4	1,249.6
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0		0.0
Reserves for Future Policy Contract Benefits		31,552.5		32,548.9
Unearned Premiums		137.9		145.1
Policy and Contract Benefits		263.7		271.4
Premium Income		995.8		994.4	1,033.5
Net Investment Income		1,159.0		1,370.3	1,404.9
Benefits and Change in Reserves for Future Benefits		1,793.2		3,426.8	2,139.3
Amortization of Deferred Acquisition Costs		0.0		0.0	0.0
All Other Expenses		284.0		240.2	232.7
Premiums Written	[4]	983.0		979.3	1,020.8
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0		0.0	0.0
Net Investment Income		27.9		9.8	20.5
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		305.3		247.7	239.5
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 benefits and change in reserves for future benefits were the following: •In 2021 and 2020, reserve increases of $2.1 million and $151.5 million, respectively, in the Closed Block segment related to reserve assumption updates for our long-term care product line. In 2021 and 2020, reserve increases of $25.1 million and $17.5 million, respectively, in the Closed Block segment related to reserve assumption updates for our group pension product line. In 2021, a reserve decrease of $215.0 million in the Unum US segment related to reserve assumption updates in our Unum US group long-term disability product line. Also in 2021, we increased reserves for our Closed Block individual disability product line by $6.4 million. •In 2021 and 2020, an increase in benefits and change in reserves for future benefits of $133.1 million and $1,284.5 million, respectively, 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, 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. Also included in all other expenses were the following: •In 2021, an impairment loss of $12.1 million was recorded in our Corporate segment for previously capitalized internal-use software that we no longer plan to utilize. •In 2021 and 2019, costs related to early retirement of debt of $67.3 million and $27.3 million, respectively, in the Corporate segment. •In 2021 and 2020, the amortization of the cost of reinsurance of $79.1 million and $2.6 million, respectively, and transaction costs of $6.2 million and $21.0 million, respectively, related to the Closed Block individual disability reinsurance transaction. •In 2021 and 2020, a right-of-use asset impairment of $13.9 million and $12.7 million, respectively, related to one of our operating leases for office space that we do not plan to continue using to support general operations in the Corporate segment. •In 2020, costs related to organizational design updates of $23.3 million in the Corporate segment.

[4] Excludes life insurance.