Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Segment Reporting Information
Premium Income		$ 10,497.4		$ 10,046.0		$ 9,616.5
Net Investment Income		2,130.0		2,096.7		2,122.2
Other Income		294.5		279.2		261.1
Commissions		1,258.6		1,170.1		1,086.4
Interest and Debt Expense		201.1		194.8		188.5
Amortization of Deferred Acquisition Costs		521.0		481.4		421.1
Unum US
Segment Reporting Information
Deferral of Acquisition Costs		(320.9)		(314.7)		(273.1)
Amortization of Deferred Acquisition Costs		292.5		267.6		240.9
Unum International
Segment Reporting Information
Deferral of Acquisition Costs		(17.8)		(14.6)		(12.0)
Amortization of Deferred Acquisition Costs		9.5		8.4		8.2
Colonial Life
Segment Reporting Information
Deferral of Acquisition Costs		(312.8)		(302.9)		(271.8)
Amortization of Deferred Acquisition Costs		219.0		205.4		172.0
Operating Segments
Segment Reporting Information
Premium Income		10,497.4		10,046.0		9,616.5
Net Investment Income		2,130.0		2,096.7		2,122.2
Other Income		294.5		279.2		261.1
Adjusted Operating Revenue		12,921.9		12,421.9		11,999.8
Adjusted Policy Benefits	[1]	7,455.1		7,277.1		7,498.3
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(204.9)	[2]	(232.0)	[2]	(294.8)	[3]
Commissions		1,258.6		1,170.1		1,086.4
Interest and Debt Expense		201.1		194.8		192.7
Deferral of Acquisition Costs		(651.5)		(632.2)		(556.9)
Amortization of Deferred Acquisition Costs		521.0		481.4		421.1
Other Segment Items		2,332.2	[4]	2,230.5	[5]	2,045.9	[6]
Adjusted Benefits and Expenses		10,911.6		10,489.7		10,392.7
Adjusted Operating Income (Loss)		2,010.3		1,932.2		1,607.1
Operating Segments | Unum US
Segment Reporting Information
Premium Income		6,883.2		6,579.2		6,251.4
Net Investment Income		632.2		639.9		676.3
Other Income		235.9		220.5		196.3
Adjusted Operating Revenue		7,751.3		7,439.6		7,124.0
Adjusted Policy Benefits	[1]	4,246.5		4,221.2		4,429.6
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(238.2)	[2]	(283.9)	[2]	(287.9)	[3]
Commissions		729.3		664.4		614.4
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(320.9)		(314.7)		(273.1)
Amortization of Deferred Acquisition Costs		292.5		267.6		240.9
Other Segment Items		1,602.9	[4]	1,529.5	[5]	1,427.5	[6]
Depreciation and Intangible Amortization		85.0		76.3		75.7
Adjusted Benefits and Expenses		6,312.1		6,084.1		6,151.4
Adjusted Operating Income (Loss)		1,439.2		1,355.5		972.6
Operating Segments | Unum International
Segment Reporting Information
Premium Income		949.5		825.2		718.8
Net Investment Income		128.8		137.2		170.1
Other Income		1.6		1.6		0.9
Adjusted Operating Revenue		1,079.9		964.0		889.8
Adjusted Policy Benefits	[1]	693.6		581.4		521.6
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(44.4)	[2]	(19.5)	[2]	35.6	[3]
Commissions		82.5		72.5		56.3
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(17.8)		(14.6)		(12.0)
Amortization of Deferred Acquisition Costs		9.5		8.4		8.2
Other Segment Items		198.7	[4]	177.7	[5]	146.1	[6]
Depreciation and Intangible Amortization		17.9		14.6		14.4
Adjusted Benefits and Expenses		922.1		805.9		755.8
Adjusted Operating Income (Loss)		157.8		158.1		134.0
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income		1,783.9		1,726.1		1,702.0
Net Investment Income		161.5		153.5		152.7
Other Income		4.0		1.2		1.1
Adjusted Operating Revenue		1,949.4		1,880.8		1,855.8
Adjusted Policy Benefits	[1]	885.4		877.1		926.6
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(34.7)	[2]	1.7	[2]	(45.3)	[3]
Commissions		378.4		359.4		340.0
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(312.8)		(302.9)		(271.8)
Amortization of Deferred Acquisition Costs		219.0		205.4		172.0
Other Segment Items		347.4	[4]	340.0	[5]	321.4	[6]
Depreciation and Intangible Amortization		15.1		13.6		15.0
Adjusted Benefits and Expenses		1,482.7		1,480.7		1,442.9
Adjusted Operating Income (Loss)		466.7		400.1		412.9
Operating Segments | Closed Block
Segment Reporting Information
Premium Income		880.8		915.5		944.3
Net Investment Income		1,148.9		1,066.3		1,070.6
Other Income		51.7		52.6		58.0
Adjusted Operating Revenue		2,081.4		2,034.4		2,072.9
Adjusted Policy Benefits	[1]	1,629.6		1,597.4		1,620.5
Adjusted Policy Benefits - Remeasurement Loss (Gain)		112.4	[2]	69.7	[2]	2.8	[3]
Commissions		68.4		73.8		75.7
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		0.0		0.0		0.0
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0
Other Segment Items		133.2	[4]	128.6	[5]	122.0	[6]
Depreciation and Intangible Amortization		5.4		5.2		4.7
Adjusted Benefits and Expenses		1,943.6		1,869.5		1,821.0
Adjusted Operating Income (Loss)		137.8		164.9		251.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income		0.0		0.0		0.0
Net Investment Income		58.6		99.8		52.5
Other Income		1.3		3.3		4.8
Adjusted Operating Revenue		59.9		103.1		57.3
Adjusted Policy Benefits	[1]	0.0		0.0		0.0
Adjusted Policy Benefits - Remeasurement Loss (Gain)		0.0	[2]	0.0	[2]	0.0	[3]
Commissions		0.0		0.0		0.0
Interest and Debt Expense		201.1		194.8		192.7
Deferral of Acquisition Costs		0.0		0.0		0.0
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0
Other Segment Items		50.0	[4]	54.7	[5]	28.9	[6]
Depreciation and Intangible Amortization		0.1		0.6		0.6
Adjusted Benefits and Expenses		251.1		249.5		221.6
Adjusted Operating Income (Loss)		$ (191.2)		$ (146.4)		$ (164.3)

[1] 1Excludes the impact of non-contemporaneous reinsurance in the Closed Block segment.

[2] 2Excludes the reserve assumption updates for each reportable segment, except for the Corporate segment, that occurred during the third quarter.

[3] 2Excludes the reserve assumption updates for each reportable segment, except for the Corporate segment, that occurred during the third quarter and the impact of non-contemporaneous reinsurance in the Closed Block segment.

[4] 3Excludes the amortization of the cost of reinsurance in the Closed Block segment and the loss on legal settlement in the Corporate segment. For each reportable segment, other segment items includes compensation, other personnel expenses, taxes, licenses and fees, depreciation, intangible asset amortization and other expenses. Depreciation and intangible asset amortization during 2024 was $85.0 million, $17.9 million, $15.1 million, $5.4 million, and $0.1 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[5] 3Excludes the amortization of the cost of reinsurance in the Closed Block segment. For each reportable segment, other segment items includes compensation, other personnel expenses, taxes, licenses and fees, depreciation, intangible asset amortization and other expenses. Depreciation and intangible asset amortization during 2023 was $76.3 million, $14.6 million, $13.6 million, $5.2 million and $0.6 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[6] 3Excludes the amortization of the cost of reinsurance in the Closed Block segment. For each reportable segment, other segment items includes compensation, other personnel expenses, taxes, licenses and fees, depreciation, intangible asset amortization and other expenses. Depreciation and intangible asset amortization during 2022 was $75.7 million, $14.4 million, $15.0 million, $4.7 million, and $0.6 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.