Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2025		Sep. 30, 2024		Sep. 30, 2025		Sep. 30, 2024
Segment Reporting Information
Premium Income	$ 2,688.0		$ 2,628.8		$ 8,138.9		$ 7,866.3
Net Investment Income	476.8		527.8		1,550.7		1,586.4
Other Income	85.6		73.3		238.3		222.5
Commissions	335.2		315.1		1,021.9		947.8
Interest and Debt Expense	52.2		49.2		156.2		148.6
Deferral of Acquisition Costs	(173.5)		(163.3)		(521.0)		(495.3)
Amortization of Deferred Acquisition Costs	132.3		133.8		389.9		387.9
Unum US
Segment Reporting Information
Amortization of Deferred Acquisition Costs					208.2		216.7
Unum International
Segment Reporting Information
Amortization of Deferred Acquisition Costs					7.8		7.3
Colonial Life
Segment Reporting Information
Amortization of Deferred Acquisition Costs					173.9		163.9
Operating Segments
Segment Reporting Information
Premium Income	2,688.0		2,628.8		8,138.9		7,866.3
Net Investment Income	476.8		527.8		1,550.7		1,586.4
Other Income	81.0	[1]	73.3		233.7	[1]	222.5
Adjusted Operating Revenue	3,245.8		3,229.9		9,923.3		9,675.2
Adjusted Policy Benefits	1,841.7	[2]	1,858.6	[3]	5,735.1	[2]	5,604.0	[3]
Adjusted Policy Benefits - Remeasurement Loss (Gain)	(2.6)	[4]	(45.3)	[5]	(60.7)	[4]	(210.1)	[5]
Commissions	335.2		315.1		1,021.9		947.8
Interest and Debt Expense	52.2		49.2		156.2		148.6
Deferral of Acquisition Costs	(173.5)		(163.3)		(521.0)		(495.3)
Amortization of Deferred Acquisition Costs	132.3		133.8		389.9		387.9
Other Segment Items	603.8	[6]	580.0	[7]	1,829.0	[8]	1,752.8	[9]
Adjusted Benefits and Expenses	2,789.1		2,728.1		8,550.4		8,135.7
Adjusted Operating Income (Loss)	456.7		501.8		1,372.9		1,539.5
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,755.4		1,723.5		5,334.9		5,161.8
Net Investment Income	151.9		161.0		455.9		476.1
Other Income	60.9	[1]	60.1		190.8	[1]	178.9
Adjusted Operating Revenue	1,968.2		1,944.6		5,981.6		5,816.8
Adjusted Policy Benefits	1,047.7	[2]	1,046.0		3,297.2	[2]	3,207.2
Adjusted Policy Benefits - Remeasurement Loss (Gain)	(4.2)	[4]	(38.3)	[5]	(98.3)	[4]	(223.6)	[5]
Commissions	195.8		182.6		605.2		551.9
Interest and Debt Expense	0.0		0.0		0.0		0.0
Deferral of Acquisition Costs	(83.3)		(81.6)		(253.7)		(247.4)
Amortization of Deferred Acquisition Costs	71.4		75.7		208.2		216.7
Other Segment Items	405.9	[6]	396.9	[7]	1,240.8	[8]	1,206.0	[9]
Adjusted Benefits and Expenses	1,633.3		1,581.3		4,999.4		4,710.8
Adjusted Operating Income (Loss)	334.9		363.3		982.2		1,106.0
Depreciation and Amortization	21.9		21.5		64.4		63.3
Operating Segments | Unum International
Segment Reporting Information
Premium Income	281.1		246.6		798.9		707.1
Net Investment Income	36.2		30.4		110.9		94.5
Other Income	2.9		0.4		3.3		1.2
Adjusted Operating Revenue	320.2		277.4		913.1		802.8
Adjusted Policy Benefits	214.7		192.7		576.9		511.9
Adjusted Policy Benefits - Remeasurement Loss (Gain)	(14.3)	[4]	(24.9)	[5]	(16.0)	[4]	(32.0)	[5]
Commissions	26.1		21.7		73.6		61.7
Interest and Debt Expense	0.0		0.0		0.0		0.0
Deferral of Acquisition Costs	(5.5)		(4.6)		(16.4)		(13.2)
Amortization of Deferred Acquisition Costs	2.7		2.5		7.8		7.3
Other Segment Items	57.7	[6]	49.7	[7]	168.1	[8]	146.9	[9]
Adjusted Benefits and Expenses	281.4		237.1		794.0		682.6
Adjusted Operating Income (Loss)	38.8		40.3		119.1		120.2
Depreciation and Amortization	5.9		4.5		15.8		13.1
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	456.5		441.9		1,375.9		1,335.0
Net Investment Income	44.3		39.6		129.1		119.4
Other Income	0.6		0.4		1.3		3.6
Adjusted Operating Revenue	501.4		481.9		1,506.3		1,458.0
Adjusted Policy Benefits	223.3		218.1		675.9		667.7
Adjusted Policy Benefits - Remeasurement Loss (Gain)	(3.1)	[4]	(7.9)	[5]	(14.4)	[4]	(27.0)	[5]
Commissions	96.7		93.0		292.6		282.2
Interest and Debt Expense	0.0		0.0		0.0		0.0
Deferral of Acquisition Costs	(84.7)		(77.1)		(250.9)		(234.7)
Amortization of Deferred Acquisition Costs	58.2		55.6		173.9		163.9
Other Segment Items	94.4	[6]	86.8	[7]	279.5	[8]	261.9	[9]
Adjusted Benefits and Expenses	384.8		368.5		1,156.6		1,114.0
Adjusted Operating Income (Loss)	116.6		113.4		349.7		344.0
Depreciation and Amortization	4.3		3.8		12.5		11.3
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	195.0		216.8		629.2		662.4
Net Investment Income	224.7		284.3		778.9		851.6
Other Income	16.3		12.4		38.3		37.7
Adjusted Operating Revenue	436.0		513.5		1,446.4		1,551.7
Adjusted Policy Benefits	356.0	[2]	401.8	[3]	1,185.1	[2]	1,217.2	[3]
Adjusted Policy Benefits - Remeasurement Loss (Gain)	19.0	[4]	25.8	[5]	68.0	[4]	72.5	[5]
Commissions	16.6		17.8		50.5		52.0
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	30.3	[6]	33.9	[7]	100.4	[8]	99.9	[9]
Adjusted Benefits and Expenses	421.9		479.3		1,404.0		1,441.6
Adjusted Operating Income (Loss)	14.1		34.2		42.4		110.1
Depreciation and Amortization	1.5		1.3		4.5		3.9
Operating Segments | Corporate
Segment Reporting Information
Premium Income	0.0		0.0		0.0		0.0
Net Investment Income	19.7		12.5		75.9		44.8
Other Income	0.3		0.0		0.0		1.1
Adjusted Operating Revenue	20.0		12.5		75.9		45.9
Adjusted Policy Benefits	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	52.2		49.2		156.2		148.6
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	15.5	[6]	12.7	[7]	40.2	[8]	38.1	[9]
Adjusted Benefits and Expenses	67.7		61.9		196.4		186.7
Adjusted Operating Income (Loss)	(47.7)		$ (49.4)		(120.5)		(140.8)
Depreciation and Amortization	$ 0.1				$ 0.4		$ 0.2

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

[4] 3Excludes the reserve assumption updates that occurred in the third quarter of 2025 for all segments except Corporate.

[5] 2Excludes the reserve assumption updates that occurred in the third quarter of 2024 for all segments except Corporate.

[6] 4Excludes 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 September 30, 2025 was $21.9 million, $5.9 million, $4.3 million, $1.5 million, and $0.1 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 the three months ended September 30, 2024 was $21.5 million, $4.5 million, $3.8 million, $1.3 million, and $0.1 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[8] 4Excludes 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 nine months ended September 30, 2025 was $64.4 million, $15.8 million, $12.5 million, $4.5 million, and $0.4 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[9] 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 the nine months ended September 30, 2024 was $63.3 million, $13.1 million, $11.3 million, $3.9 million, and $0.2 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.