Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026		Mar. 31, 2025
Segment Reporting
Premium Income	$ 2,794.0		$ 2,702.9
Net Investment Income	483.4		513.2
Other Income	82.8		82.3
Benefits and Change in Reserves for Future Benefits	1,955.8		1,960.3
Policy Benefits - Remeasurement Loss (Gain)	48.2		(89.3)
Commissions	368.5		343.2
Interest and Debt Expense	53.1		52.0
Deferral of Acquisition Costs	(190.6)		(172.6)
Amortization of Deferred Acquisition Costs	134.2		125.4
Benefits, Losses and Expenses	3,052.5		2,848.0
Unum US
Segment Reporting
Amortization of Deferred Acquisition Costs	68.7		65.2
Unum International
Segment Reporting
Amortization of Deferred Acquisition Costs	3.4		2.5
Colonial Life
Segment Reporting
Amortization of Deferred Acquisition Costs	62.1		57.7
Operating Segments
Segment Reporting
Premium Income	2,794.0		2,702.9
Net Investment Income	483.4		513.2
Other Income	82.8		82.3
Segment Adjusted Operating Revenue	3,360.2		3,298.4
Benefits and Change in Reserves for Future Benefits	1,955.8		1,960.3
Policy Benefits - Remeasurement Loss (Gain)	48.2		(89.3)
Commissions	368.5		343.2
Interest and Debt Expense	53.1		52.0
Deferral of Acquisition Costs	(190.6)		(172.6)
Amortization of Deferred Acquisition Costs	134.2		125.4
Benefits, Losses and Expenses	3,052.5		2,848.0
Segment Adjusted Operating Income (Loss)	307.7		450.4
Other Segment Items	683.3	[1]	629.0	[2]
Operating Segments | Unum US
Segment Reporting
Premium Income	1,840.5		1,780.9
Net Investment Income	158.3		148.9
Other Income	66.8		71.9
Segment Adjusted Operating Revenue	2,065.6		2,001.7
Benefits and Change in Reserves for Future Benefits	1,127.1		1,138.6
Policy Benefits - Remeasurement Loss (Gain)	(31.8)		(75.4)
Commissions	219.4		205.6
Interest and Debt Expense	0.0
Deferral of Acquisition Costs	(91.6)		(84.7)
Amortization of Deferred Acquisition Costs	68.7		65.2
Benefits, Losses and Expenses	1,727.7		1,672.6
Segment Adjusted Operating Income (Loss)	337.9		329.1
Other Segment Items	435.9	[1]	423.3	[2]
Depreciation and Amortization	21.7		21.3
Operating Segments | Unum International
Segment Reporting
Premium Income	286.7		246.7
Net Investment Income	29.6		28.5
Other Income	3.3		0.1
Segment Adjusted Operating Revenue	319.6		275.3
Benefits and Change in Reserves for Future Benefits	206.3		172.9
Policy Benefits - Remeasurement Loss (Gain)	(2.8)		(8.8)
Commissions	31.0		22.4
Interest and Debt Expense	0.0		0.0
Deferral of Acquisition Costs	(9.2)		(5.3)
Amortization of Deferred Acquisition Costs	3.4		2.5
Benefits, Losses and Expenses	288.7		236.6
Segment Adjusted Operating Income (Loss)	30.9		38.7
Other Segment Items	60.0	[1]	52.9	[2]
Depreciation and Amortization	5.5		4.7
Operating Segments | Colonial Life
Segment Reporting
Premium Income	472.7		457.3
Net Investment Income	46.8		42.2
Other Income	0.7		0.4
Segment Adjusted Operating Revenue	520.2		499.9
Benefits and Change in Reserves for Future Benefits	229.7		226.6
Policy Benefits - Remeasurement Loss (Gain)	(12.4)		(8.5)
Commissions	102.9		97.3
Interest and Debt Expense	0.0		0.0
Deferral of Acquisition Costs	(89.8)		(82.6)
Amortization of Deferred Acquisition Costs	62.1		57.7
Benefits, Losses and Expenses	392.4		384.2
Segment Adjusted Operating Income (Loss)	127.8		115.7
Other Segment Items	99.9	[1]	93.7	[2]
Depreciation and Amortization	4.4		4.2
Operating Segments | Closed Block
Segment Reporting
Premium Income	194.1		218.0
Net Investment Income	229.1		269.7
Other Income	11.3		9.9
Segment Adjusted Operating Revenue	434.5		497.6
Benefits and Change in Reserves for Future Benefits	392.7		422.2
Policy Benefits - Remeasurement Loss (Gain)	95.2		3.4
Commissions	15.2		17.9
Interest and Debt Expense	0.0		0.0
Deferral of Acquisition Costs	0.0		0.0
Amortization of Deferred Acquisition Costs	0.0		0.0
Benefits, Losses and Expenses	579.8		489.6
Segment Adjusted Operating Income (Loss)	(145.3)		8.0
Other Segment Items	76.7	[1]	46.1	[2]
Depreciation and Amortization	1.6		1.5
Operating Segments | Corporate
Segment Reporting
Premium Income	0.0		0.0
Net Investment Income	19.6		23.9
Other Income	0.7		0.0
Segment Adjusted Operating Revenue	20.3		23.9
Benefits and Change in Reserves for Future Benefits	0.0		0.0
Policy Benefits - Remeasurement Loss (Gain)	0.0		0.0
Commissions	0.0		0.0
Interest and Debt Expense	53.1		52.0
Deferral of Acquisition Costs	0.0		0.0
Amortization of Deferred Acquisition Costs	0.0		0.0
Benefits, Losses and Expenses	63.9		65.0
Segment Adjusted Operating Income (Loss)	(43.6)		(41.1)
Other Segment Items	10.8	[1]	13.0	[2]
Depreciation and Amortization	$ 0.2		$ 0.1

[1] 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 March 31, 2026 was $21.7 million, $5.5 million, $4.4 million, $1.6 million, and $0.2 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 March 31, 2025 was $21.3 million, $4.7 million, $4.2 million, $1.5 million, and $0.1 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.