Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2026		Jun. 30, 2025		Jun. 30, 2026		Jun. 30, 2025
Segment Reporting
Premium Income		$ 2,817.8		$ 2,748.0		$ 5,611.8		$ 5,450.9
Net Investment Income		478.4		560.7		961.8		1,073.9
Other Income		79.0		70.4		161.8		152.7
Policy Benefits		1,991.2		1,944.9		3,947.0		3,905.2
Policy Benefits - Remeasurement Loss (Gain)		10.0		31.2		58.2		(58.1)
Commissions		364.3		343.5		732.8		686.7
Interest and Debt Expense		53.3		52.0		106.4		104.0
Deferral of Acquisition Costs		(187.4)		(174.9)		(378.0)		(347.5)
Amortization of Deferred Acquisition Costs		135.4		132.2		269.6		257.6
Unum US
Segment Reporting
Amortization of Deferred Acquisition Costs						139.6		136.8
Unum International
Segment Reporting
Amortization of Deferred Acquisition Costs						6.1		5.1
Colonial Life
Segment Reporting
Amortization of Deferred Acquisition Costs						123.9		115.7
Operating Segments
Segment Reporting
Premium Income		2,817.8		2,748.0		5,611.8		5,450.9
Adjusted Net Investment Income		494.7				978.1
Net Investment Income				560.7				1,073.9
Other Income		79.0		70.4		161.8		152.7
Segment Adjusted Operating Revenue		3,391.5		3,379.1		6,751.7		6,677.5
Policy Benefits		1,991.2		1,944.9		3,947.0		3,905.2
Policy Benefits - Remeasurement Loss (Gain)		10.0		31.2		58.2		(58.1)
Commissions		364.3		343.5		732.8		686.7
Interest and Debt Expense		53.3		52.0		106.4		104.0
Deferral of Acquisition Costs		(187.4)		(174.9)		(378.0)		(347.5)
Amortization of Deferred Acquisition Costs		135.4		132.2		269.6		257.6
Other Segment Items		659.3	[1]	615.5	[2]	1,342.6	[3]	1,244.5	[4]
Adjusted Benefits and Expenses		3,026.1		2,944.4		6,078.6		5,792.4
Adjusted Operating Income (Loss)		365.4		434.7		673.1		885.1
Operating Segments | Unum US
Segment Reporting
Premium Income		1,858.2		1,798.6		3,698.7		3,579.5
Adjusted Net Investment Income		155.8				314.1
Net Investment Income				155.1				304.0
Other Income		65.9		58.0		132.7		129.9
Segment Adjusted Operating Revenue		2,079.9		2,011.7		4,145.5		4,013.4
Policy Benefits		1,165.1		1,110.9		2,292.2		2,249.5
Policy Benefits - Remeasurement Loss (Gain)		(29.1)		(18.7)		(60.9)		(94.1)
Commissions		213.7		203.8		433.1		409.4
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(89.7)		(85.7)		(181.3)		(170.4)
Amortization of Deferred Acquisition Costs		70.9		71.6		139.6		136.8
Other Segment Items		419.4	[1]	411.6	[2]	855.3	[3]	834.9	[4]
Adjusted Benefits and Expenses		1,750.3		1,693.5		3,478.0		3,366.1
Adjusted Operating Income (Loss)		329.6		318.2		667.5		647.3
Depreciation and Amortization		21.0		21.2	[2]	42.7		42.5
Operating Segments | Unum International
Segment Reporting
Premium Income		289.3		271.1		576.0		517.8
Adjusted Net Investment Income		42.2				71.8
Net Investment Income				46.2				74.7
Other Income		2.7		0.3		6.0		0.4
Segment Adjusted Operating Revenue		334.2		317.6		653.8		592.9
Policy Benefits		219.8		189.3		426.1		362.2
Policy Benefits - Remeasurement Loss (Gain)		6.9		7.1		4.1		(1.7)
Commissions		29.2		25.1		60.2		47.5
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(7.1)		(5.6)		(16.3)		(10.9)
Amortization of Deferred Acquisition Costs		2.7		2.6		6.1		5.1
Other Segment Items		58.4	[1]	57.5	[2]	118.4	[3]	110.4	[4]
Adjusted Benefits and Expenses		309.9		276.0		598.6		512.6
Adjusted Operating Income (Loss)		24.3		41.6		55.2		80.3
Depreciation and Amortization		6.5		5.2	[2]	12.0		9.9
Operating Segments | Colonial Life
Segment Reporting
Premium Income		477.4		462.1		950.1		919.4
Adjusted Net Investment Income		50.1				96.9
Net Investment Income				42.6				84.8
Other Income		0.4		0.3		1.1		0.7
Segment Adjusted Operating Revenue		527.9		505.0		1,048.1		1,004.9
Policy Benefits		230.9		226.0		460.6		452.6
Policy Benefits - Remeasurement Loss (Gain)		(8.0)		(2.8)		(20.4)		(11.3)
Commissions		106.9		98.6		209.8		195.9
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(90.6)		(83.6)		(180.4)		(166.2)
Amortization of Deferred Acquisition Costs		61.8		58.0		123.9		115.7
Other Segment Items		95.5	[1]	91.4	[2]	195.4	[3]	185.1	[4]
Adjusted Benefits and Expenses		396.5		387.6		788.9		771.8
Adjusted Operating Income (Loss)		131.4		117.4		259.2		233.1
Depreciation and Amortization		4.4		4.0	[2]	8.8		8.2
Operating Segments | Closed Block
Segment Reporting
Premium Income		192.9		216.2		387.0		434.2
Adjusted Net Investment Income		226.1				455.2
Net Investment Income				284.5				554.2
Other Income		9.6		12.1		20.9		22.0
Segment Adjusted Operating Revenue		428.6		512.8		863.1		1,010.4
Policy Benefits		373.1		418.7		765.8		840.9
Policy Benefits - Remeasurement Loss (Gain)		40.2		45.6		135.4		49.0
Commissions		14.5		16.0		29.7		33.9
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		0.0		0.0		0.0		0.0
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0		0.0
Other Segment Items		76.2	[1]	43.3	[2]	152.9	[3]	89.4	[4]
Adjusted Benefits and Expenses		504.0		523.6		1,083.8		1,013.2
Adjusted Operating Income (Loss)		(75.4)		(10.8)		(220.7)		(2.8)
Depreciation and Amortization		1.6		1.5	[2]	3.2		3.0
Operating Segments | Corporate
Segment Reporting
Premium Income		0.0		0.0		0.0		0.0
Adjusted Net Investment Income	[5]	20.5				40.1
Net Investment Income				32.3				56.2
Other Income		0.4		(0.3)		1.1		(0.3)
Segment Adjusted Operating Revenue		20.9		32.0		41.2		55.9
Policy Benefits		2.3		0.0		2.3		0.0
Policy Benefits - Remeasurement Loss (Gain)		0.0		0.0		0.0		0.0
Commissions		0.0		0.0				0.0
Interest and Debt Expense		53.3		52.0		106.4		104.0
Deferral of Acquisition Costs		0.0		0.0		0.0		0.0
Amortization of Deferred Acquisition Costs		0.0		0.0		0.0		0.0
Other Segment Items		9.8	[1],[5]	11.7	[2]	20.6	[3],[5]	24.7	[4]
Adjusted Benefits and Expenses		65.4		63.7		129.3		128.7
Adjusted Operating Income (Loss)		(44.5)		$ (31.7)		(88.1)		(72.8)
Depreciation and Amortization		$ 17.6				$ 18.9		$ 0.3

[1] 2For 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 the three months ended June 30, 2026 was $21.0 million, $6.5 million, $4.4 million, $1.6 million, and $17.6 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[2] 1For 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 the three months ended June 30, 2025 was $21.2 million, $5.2 million, $4.0 million, $1.5 million, and $0.2 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[3] 2For 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 the six months ended June 30, 2026 was $42.7 million, $12.0 million, $8.8 million, $3.2 million, and $18.9 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[4] 1For 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 the six months ended June 30, 2025 was $42.5 million, $9.9 million, $8.2 million, $3.0 million, and $0.3 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[5] 1Excludes the impact of the strategic actions in the Corporate segment.