Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025		Dec. 31, 2024		Dec. 31, 2023
Segment Reporting Information
Premium Income		$ 10,831.0		$ 10,497.4		$ 10,046.0
Net Investment Income		2,032.7		2,130.0		2,096.7
Other Income		318.4		294.5		279.2
Commissions		1,355.3		1,258.6		1,170.1
Interest and Debt Expense		208.8		201.1		194.8
Deferral of Acquisition Costs		(697.1)		(651.5)		(632.2)
Amortization of Deferred Acquisition Costs		527.1		521.0		481.4
Unum US
Segment Reporting Information
Amortization of Deferred Acquisition Costs		283.3		292.5		267.6
Unum International
Segment Reporting Information
Amortization of Deferred Acquisition Costs		10.7		9.5		8.4
Colonial Life
Segment Reporting Information
Amortization of Deferred Acquisition Costs		233.1		219.0		205.4
Operating Segments
Segment Reporting Information
Premium Income		10,831.0		10,497.4		10,046.0
Net Investment Income		2,032.7		2,130.0		2,096.7
Adjusted Other Income	[1]	309.4
Other Income				294.5		279.2
Adjusted Operating Revenue		13,173.1		12,921.9		12,421.9
Adjusted Policy Benefits	[2]	7,596.6		7,455.1		7,277.1
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(39.4)	[3]	(204.9)	[4]	(232.0)	[5]
Commissions		1,355.3		1,258.6		1,170.1
Interest and Debt Expense		208.8		201.1		194.8
Deferral of Acquisition Costs		(697.1)		(651.5)		(632.2)
Amortization of Deferred Acquisition Costs		527.1		521.0		481.4
Other Segment Items		2,442.1	[6]	2,332.2	[7]	2,230.5	[8]
Adjusted Benefits and Expenses		11,393.4		10,911.6		10,489.7
Adjusted Operating Income (Loss)		1,779.7		2,010.3		1,932.2
Operating Segments | Unum US
Segment Reporting Information
Premium Income		7,083.5		6,883.2		6,579.2
Net Investment Income		604.2		632.2		639.9
Adjusted Other Income	[1]	249.8
Other Income				235.9		220.5
Adjusted Operating Revenue		7,937.5		7,751.3		7,439.6
Adjusted Policy Benefits		4,353.3	[2]	4,246.5		4,221.2
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(86.2)	[3]	(238.2)	[4]	(283.9)	[5]
Commissions		794.9		729.3		664.4
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(331.7)		(320.9)		(314.7)
Amortization of Deferred Acquisition Costs		283.3		292.5		267.6
Other Segment Items		1,652.0	[6]	1,602.9	[7]	1,529.5	[8]
Depreciation and Intangible Amortization		86.7		85.0		76.3
Adjusted Benefits and Expenses		6,665.6		6,312.1		6,084.1
Adjusted Operating Income (Loss)		1,271.9		1,439.2		1,355.5
Operating Segments | Unum International
Segment Reporting Information
Premium Income		1,082.8		949.5		825.2
Net Investment Income		145.6		128.8		137.2
Adjusted Other Income		7.7
Other Income				1.6		1.6
Adjusted Operating Revenue		1,236.1		1,079.9		964.0
Adjusted Policy Benefits		793.0		693.6		581.4
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(20.2)	[3]	(44.4)	[4]	(19.5)	[5]
Commissions		95.7		82.5		72.5
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(22.0)		(17.8)		(14.6)
Amortization of Deferred Acquisition Costs		10.7		9.5		8.4
Other Segment Items		226.6	[6]	198.7	[7]	177.7	[8]
Depreciation and Intangible Amortization		20.7		17.9		14.6
Adjusted Benefits and Expenses		1,083.8		922.1		805.9
Adjusted Operating Income (Loss)		152.3		157.8		158.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income		1,839.1		1,783.9		1,726.1
Net Investment Income		172.6		161.5		153.5
Adjusted Other Income		1.6
Other Income				4.0		1.2
Adjusted Operating Revenue		2,013.3		1,949.4		1,880.8
Adjusted Policy Benefits		908.1		885.4		877.1
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(23.0)	[3]	(34.7)	[4]	1.7	[5]
Commissions		398.6		378.4		359.4
Interest and Debt Expense		0.0		0.0		0.0
Deferral of Acquisition Costs		(343.4)		(312.8)		(302.9)
Amortization of Deferred Acquisition Costs		233.1		219.0		205.4
Other Segment Items		376.3	[6]	347.4	[7]	340.0	[8]
Depreciation and Intangible Amortization		16.9		15.1		13.6
Adjusted Benefits and Expenses		1,549.7		1,482.7		1,480.7
Adjusted Operating Income (Loss)		463.6		466.7		400.1
Operating Segments | Closed Block
Segment Reporting Information
Premium Income		825.6		880.8		915.5
Net Investment Income		1,016.5		1,148.9		1,066.3
Adjusted Other Income		49.9
Other Income				51.7		52.6
Adjusted Operating Revenue		1,892.0		2,081.4		2,034.4
Adjusted Policy Benefits		1,542.2	[2]	1,629.6	[9]	1,597.4	[9]
Adjusted Policy Benefits - Remeasurement Loss (Gain)		90.0	[3]	112.4	[4]	69.7	[5]
Commissions		66.1		68.4		73.8
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		130.2	[6]	133.2	[7]	128.6	[8]
Depreciation and Intangible Amortization		6.0		5.4		5.2
Adjusted Benefits and Expenses		1,828.5		1,943.6		1,869.5
Adjusted Operating Income (Loss)		63.5		137.8		164.9
Operating Segments | Corporate and Other Segment
Segment Reporting Information
Premium Income		0.0		0.0		0.0
Net Investment Income		93.8		58.6		99.8
Adjusted Other Income		0.4
Other Income				1.3		3.3
Adjusted Operating Revenue		94.2		59.9		103.1
Adjusted Policy Benefits		0.0		0.0		0.0
Adjusted Policy Benefits - Remeasurement Loss (Gain)		0.0		0.0		0.0
Commissions		0.0		0.0		0.0
Interest and Debt Expense		208.8		201.1		194.8
Deferral of Acquisition Costs		0.0		0.0		0.0
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0
Other Segment Items		57.0	[6]	50.0	[7]	54.7	[8]
Depreciation and Intangible Amortization		0.5		0.1		0.6
Adjusted Benefits and Expenses		265.8		251.1		249.5
Adjusted Operating Income (Loss)		$ (171.6)		$ (191.2)		$ (146.4)

[1] 1Excludes the amortization of the deferred gain on reinsurance in the Unum US segment.

[2] 2Excludes the impact of non-contemporaneous reinsurance in the Unum US segment and the Closed Block segment.

[3] 3Excludes the reserve assumption updates that occurred in the third quarter of 2025 for all segments except Corporate.

[4] 2Excludes the reserve assumption updates that occurred in the third quarter of 2024 for all segments except Corporate.

[5] 2Excludes the reserve assumption updates that occurred in the third quarter of 2023 for all segments except Corporate.

[6] 4Excludes the amortization of the cost of reinsurance in the Closed Block segment, as well as the accelerated charitable contribution and the settlement loss on the U.S. pension plan annuity purchase 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 2025 was $86.7 million, $20.7 million, $16.9 million, $6.0 million, and $0.5 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[7] 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.

[8] 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.

[9] 1Excludes the impact of non-contemporaneous reinsurance in the Closed Block segment.