Schedule III Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		$ 2,252.3		$ 2,207.9
Reserves for Future Policy Contract Benefits		42,330.2		48,007.5
Unearned Premiums		352.7		347.5
Policy and Contract Benefits		1,839.8		1,907.7
Premium Income		9,623.4		9,481.0		$ 9,378.1
Net Investment Income	[1]	2,122.2		2,213.2		2,360.7
Benefits and Change in Reserves for Future Benefits		6,936.7	[2]	7,598.6		8,972.9	[2]
Amortization of Deferred Acquisition Costs		591.0		586.1		606.1
All Other Expenses	[3]	2,831.9		2,766.1		2,619.1
Unum US
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,089.4		1,096.2
Reserves for Future Policy Contract Benefits		9,129.9		10,959.4
Unearned Premiums		50.5		41.5
Policy and Contract Benefits		1,245.8		1,262.5
Premium Income		6,258.3		6,078.0		6,018.9
Net Investment Income	[1]	676.3		721.6		720.3
Benefits and Change in Reserves for Future Benefits		3,941.5	[2]	4,338.8	[2]	4,138.7
Amortization of Deferred Acquisition Costs		294.9		319.0		341.0
All Other Expenses		1,768.8		1,631.9		1,589.0
Premiums Written	[4]	4,335.7		4,168.9		4,088.6
Unum International
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		36.6		35.4
Reserves for Future Policy Contract Benefits		2,074.7		2,765.5
Unearned Premiums		130.4		131.8
Policy and Contract Benefits		165.2		171.2
Premium Income		718.8		717.0		652.8
Net Investment Income	[1]	170.1		132.7		104.6
Benefits and Change in Reserves for Future Benefits		564.8		556.2		500.9
Amortization of Deferred Acquisition Costs		7.6		8.0		7.4
All Other Expenses		190.4		180.4		173.0
Premiums Written	[4]	504.1		520.6		456.0
Colonial Life
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		1,126.3		1,076.3
Reserves for Future Policy Contract Benefits		2,750.2		2,730.1
Unearned Premiums		35.2		36.3
Policy and Contract Benefits		181.5		210.3
Premium Income		1,702.0		1,690.2		1,712.0
Net Investment Income	[1]	152.7		172.0		155.7
Benefits and Change in Reserves for Future Benefits		803.1		910.4		906.5
Amortization of Deferred Acquisition Costs		288.5		259.1		257.7
All Other Expenses		389.6		364.5		369.2
Premiums Written	[4]	1,221.8		1,223.3		1,252.4
Closed Block
Supplementary Insurance Information, by Segment
Deferred Acquisition Costs		0.0		0.0
Reserves for Future Policy Contract Benefits		28,375.4		31,552.5
Unearned Premiums		136.6		137.9
Policy and Contract Benefits		247.3		263.7
Premium Income		944.3		995.8		994.4
Net Investment Income	[1]	1,070.6		1,159.0		1,370.3
Benefits and Change in Reserves for Future Benefits		1,627.3		1,793.2	[2]	3,426.8	[2]
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0
All Other Expenses		261.5		284.0		240.2
Premiums Written	[4]	936.8		983.0		979.3
Corporate and Other Segment
Supplementary Insurance Information, by Segment
Premium Income		0.0		0.0		0.0
Net Investment Income	[1]	52.5		27.9		9.8
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		221.6		305.3		247.7
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 2022, a reserve decrease in the Unum US segment related to reserve assumption updates in our Unum US group long-term disability and group life product lines, of $121.0 million and $34.0 million, respectively. •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, reserve increases in the Closed Block segment related to reserve assumption updates for our long-term care, group pension, and individual disability product lines, of $2.1 million, $25.1 million, and $6.4 million, respectively. •In 2020, reserve increases in the Closed Block segment related to reserve assumption updates for our long-term care and group pension product lines of $151.5 million and $17.5 million, respectively. •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 2022, 2021, and 2020 the amortization of the cost of reinsurance of $63.8 million, $79.1 million, and $2.6 million respectively, in the Closed Block segment. •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, costs related to early retirement of debt of $67.3 million in the Corporate segment. •In 2021 and 2020, 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.