Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2025		Jun. 30, 2024		Jun. 30, 2025		Jun. 30, 2024
Segment Reporting Information
Premium Income		$ 2,748.0		$ 2,627.2		$ 5,450.9		$ 5,237.5
Net Investment Income		560.7		545.1		1,073.9		1,058.6
Other Income		70.4		71.5		152.7		149.2
Commissions		343.5		319.1		686.7		632.7
Interest and Debt Expense		52.0		49.9		104.0		99.4
Deferral of Acquisition Costs		(174.9)		(165.1)		(347.5)		(332.0)
Amortization of Deferred Acquisition Costs		132.2		127.9		257.6		254.1
Unum US
Segment Reporting Information
Amortization of Deferred Acquisition Costs						136.8		141.0
Unum International
Segment Reporting Information
Amortization of Deferred Acquisition Costs						5.1		4.8
Colonial Life
Segment Reporting Information
Amortization of Deferred Acquisition Costs						115.7		108.3
Operating Segments
Segment Reporting Information
Premium Income		2,748.0		2,627.2		5,450.9		5,237.5
Net Investment Income		560.7		545.1		1,073.9		1,058.6
Other Income		70.4		71.5		152.7		149.2
Adjusted Operating Revenue		3,379.1		3,243.8		6,677.5		6,445.3
Adjusted Policy Benefits	[1]	1,939.9		1,859.6		3,893.4		3,745.4
Adjusted Policy Benefits - Remeasurement Loss (Gain)		31.2		(57.1)		(58.1)		(164.8)
Commissions		343.5		319.1		686.7		632.7
Interest and Debt Expense		52.0		49.9		104.0		99.4
Deferral of Acquisition Costs		(174.9)		(165.1)		(347.5)		(332.0)
Amortization of Deferred Acquisition Costs		132.2		127.9		257.6		254.1
Other Segment Items		605.8	[2]	586.3	[3]	1,225.2	[4]	1,172.8	[5]
Adjusted Benefits and Expenses		2,929.7		2,720.6		5,761.3		5,407.6
Adjusted Operating Income (Loss)		449.4		523.2		916.2		1,037.7
Operating Segments | Unum US
Segment Reporting Information
Premium Income		1,798.6		1,730.9		3,579.5		3,438.3
Net Investment Income		155.1		158.1		304.0		315.1
Other Income		58.0		58.2		129.9		118.8
Adjusted Operating Revenue		2,011.7		1,947.2		4,013.4		3,872.2
Adjusted Policy Benefits	[1]	1,110.9		1,079.5		2,249.5		2,161.2
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(18.7)		(72.9)		(94.1)		(185.3)
Commissions		203.8		187.1		409.4		369.3
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(85.7)		(82.5)		(170.4)		(165.8)
Amortization of Deferred Acquisition Costs		71.6		71.2		136.8		141.0
Other Segment Items		411.6	[2]	407.3	[3]	834.9	[4]	809.1	[5]
Depreciation and Amortization		21.2		20.5		42.5		41.8
Adjusted Benefits and Expenses		1,693.5		1,589.7		3,366.1		3,129.5
Adjusted Operating Income (Loss)		318.2		357.5		647.3		742.7
Operating Segments | Unum International
Segment Reporting Information
Premium Income		271.1		228.8		517.8		460.5
Net Investment Income		46.2		38.0		74.7		64.1
Other Income		0.3		0.5		0.4		0.8
Adjusted Operating Revenue		317.6		267.3		592.9		525.4
Adjusted Policy Benefits	[1]	189.3		155.3		362.2		319.2
Adjusted Policy Benefits - Remeasurement Loss (Gain)		7.1		0.9		(1.7)		(7.1)
Commissions		25.1		20.4		47.5		40.0
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(5.6)		(4.3)		(10.9)		(8.6)
Amortization of Deferred Acquisition Costs		2.6		2.4		5.1		4.8
Other Segment Items		57.5	[2]	50.1	[3]	110.4	[4]	97.2	[5]
Depreciation and Amortization		5.2		4.3		9.9		8.6
Adjusted Benefits and Expenses		276.0		224.8		512.6		445.5
Adjusted Operating Income (Loss)		41.6		42.5		80.3		79.9
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income		462.1		446.2		919.4		893.1
Net Investment Income		42.6		40.5		84.8		79.8
Other Income		0.3		0.2		0.7		3.2
Adjusted Operating Revenue		505.0		486.9		1,004.9		976.1
Adjusted Policy Benefits	[1]	226.0		222.9		452.6		449.6
Adjusted Policy Benefits - Remeasurement Loss (Gain)		(2.8)		(9.5)		(11.3)		(19.1)
Commissions		98.6		94.0		195.9		189.2
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(83.6)		(78.3)		(166.2)		(157.6)
Amortization of Deferred Acquisition Costs		58.0		54.3		115.7		108.3
Other Segment Items		91.4	[2]	86.6	[3]	185.1	[4]	175.1	[5]
Depreciation and Amortization		4.0		3.7		8.2		7.5
Adjusted Benefits and Expenses		387.6		370.0		771.8		745.5
Adjusted Operating Income (Loss)		117.4		116.9		233.1		230.6
Operating Segments | Closed Block
Segment Reporting Information
Premium Income		216.2		221.3		434.2		445.6
Net Investment Income		284.5		294.2		554.2		567.3
Other Income		12.1		12.2		22.0		25.3
Adjusted Operating Revenue		512.8		527.7		1,010.4		1,038.2
Adjusted Policy Benefits	[1]	413.7		401.9		829.1		815.4
Adjusted Policy Benefits - Remeasurement Loss (Gain)		45.6		24.4		49.0		46.7
Commissions		16.0		17.6		33.9		34.2
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		33.6	[2]	32.2	[3]	70.1	[4]	66.0	[5]
Depreciation and Amortization		1.5		1.3		3.0		2.6
Adjusted Benefits and Expenses		508.9		476.1		982.1		962.3
Adjusted Operating Income (Loss)		3.9		51.6		28.3		75.9
Operating Segments | Corporate
Segment Reporting Information
Premium Income		0.0		0.0		0.0		0.0
Net Investment Income		32.3		14.3		56.2		32.3
Other Income		(0.3)		0.4		(0.3)		1.1
Adjusted Operating Revenue		32.0		14.7		55.9		33.4
Adjusted Policy Benefits	[1]	0.0		0.0		0.0		0.0
Adjusted Policy Benefits - Remeasurement Loss (Gain)		0.0		0.0		0.0		0.0
Commissions		0.0		0.0				0.0
Interest and Debt Expense		52.0		49.9		104.0		99.4
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		11.7	[2]	10.1	[3]	24.7	[4]	25.4	[5]
Depreciation and Amortization		0.2				0.3		0.1
Adjusted Benefits and Expenses		63.7		60.0		128.7		124.8
Adjusted Operating Income (Loss)		$ (31.7)		$ (45.3)		$ (72.8)		$ (91.4)

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

[2] 2Excludes 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 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] 2Excludes 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 the three months ended June 30, 2024 was $20.5 million, $4.3 million, $3.7 million, $1.3 million, and de minimis for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[4] 2Excludes 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 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] 2Excludes 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 the six months ended June 30, 2024 was $41.8 million, $8.6 million, $7.5 million, $2.6 million, and $0.1 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.