Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2026		Sep. 30, 2025		Sep. 30, 2026		Sep. 30, 2025
Segment Reporting
Premium Income		$ 2,800.7		$ 2,688.0		$ 8,412.5		$ 8,138.9
Net Investment Income		522.4		476.8		1,484.2		1,550.7
Other Income		77.4		85.6		239.2		238.3
Policy Benefits		2,021.4		1,849.4		5,968.4		5,754.6
Policy Benefits - Remeasurement Loss (Gain)		53.3		475.9		111.5		417.8
Commissions		365.0		335.2		1,097.8		1,021.9
Interest and Debt Expense		54.0		52.2		160.4		156.2
Deferral of Acquisition Costs		(188.3)		(173.5)		(566.3)		(521.0)
Amortization of Deferred Acquisition Costs		144.3		132.3		413.9		389.9
Unum US
Segment Reporting
Amortization of Deferred Acquisition Costs						216.8		208.2
Unum International
Segment Reporting
Amortization of Deferred Acquisition Costs						9.7		7.8
Colonial Life
Segment Reporting
Amortization of Deferred Acquisition Costs						187.4		173.9
Operating Segments
Segment Reporting
Premium Income		2,800.7		2,688.0		8,412.5		8,138.9
Net Investment Income		522.4		476.8				1,550.7
Adjusted Net Investment Income						1,500.5
Other Income		77.4		85.6		239.2		238.3
Segment Adjusted Operating Revenue		3,400.5		3,250.4		10,152.2		9,927.9
Policy Benefits		2,021.4		1,849.4		5,968.4		5,754.6
Policy Benefits - Remeasurement Loss (Gain)		53.3		475.9		111.5		417.8
Commissions		365.0		335.2		1,097.8		1,021.9
Interest and Debt Expense		54.0		52.2		160.4		156.2
Deferral of Acquisition Costs		(188.3)		(173.5)		(566.3)		(521.0)
Amortization of Deferred Acquisition Costs		144.3		132.3		413.9		389.9
Other Segment Items		647.2	[1]	652.4	[2]	1,989.8	[3],[4]	1,896.9	[5]
Adjusted Benefits and Expenses		3,096.9		3,323.9		9,175.5		9,116.3
Adjusted Operating Income (Loss)		303.6		(73.5)		976.7		811.6
Operating Segments | Unum US
Segment Reporting
Premium Income		1,846.6		1,755.4		5,545.3		5,334.9
Net Investment Income		164.9		151.9				455.9
Adjusted Net Investment Income						479.0
Other Income		65.2		65.5		197.9		195.4
Segment Adjusted Operating Revenue		2,076.7		1,972.8		6,222.2		5,986.2
Policy Benefits		1,191.0		1,048.1		3,483.2		3,297.6
Policy Benefits - Remeasurement Loss (Gain)		(46.3)		(151.9)		(107.2)		(246.0)
Commissions		219.3		195.8		652.4		605.2
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(94.2)		(83.3)		(275.5)		(253.7)
Amortization of Deferred Acquisition Costs		77.2		71.4		216.8		208.2
Other Segment Items		414.4	[1]	405.9	[2]	1,269.7	[4]	1,240.8	[5]
Adjusted Benefits and Expenses		1,761.4		1,486.0		5,239.4		4,852.1
Adjusted Operating Income (Loss)		315.3		486.8		982.8		1,134.1
Depreciation and Amortization		21.4		21.9	[2]	64.1		64.4
Operating Segments | Unum International
Segment Reporting
Premium Income		291.6		281.1		867.6		798.9
Net Investment Income		37.4		36.2				110.9
Adjusted Net Investment Income						109.2
Other Income		2.1		2.9		8.1		3.3
Segment Adjusted Operating Revenue		331.1		320.2		984.9		913.1
Policy Benefits		219.2		214.7		645.3		576.9
Policy Benefits - Remeasurement Loss (Gain)		32.5		(19.7)		36.6		(21.4)
Commissions		28.5		26.1		88.7		73.6
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(6.8)		(5.5)		(23.1)		(16.4)
Amortization of Deferred Acquisition Costs		3.6		2.7		9.7		7.8
Other Segment Items		53.9	[1]	57.7	[2]	172.3	[4]	168.1	[5]
Adjusted Benefits and Expenses		330.9		276.0		929.5		788.6
Adjusted Operating Income (Loss)		0.2		44.2		55.4		124.5
Depreciation and Amortization		6.1		5.9	[2]	18.1		15.8
Operating Segments | Colonial Life
Segment Reporting
Premium Income		470.0		456.5		1,420.1		1,375.9
Net Investment Income		50.3		44.3				129.1
Adjusted Net Investment Income						147.2
Other Income		0.5		0.6		1.6		1.3
Segment Adjusted Operating Revenue		520.8		501.4		1,568.9		1,506.3
Policy Benefits		226.0		223.3		686.6		675.9
Policy Benefits - Remeasurement Loss (Gain)		(47.5)		(12.0)		(67.9)		(23.3)
Commissions		102.8		96.7		312.6		292.6
Interest and Debt Expense		0.0		0.0		0.0		0.0
Deferral of Acquisition Costs		(87.3)		(84.7)		(267.7)		(250.9)
Amortization of Deferred Acquisition Costs		63.5		58.2		187.4		173.9
Other Segment Items		93.9	[1]	94.4	[2]	289.3	[4]	279.5	[5]
Adjusted Benefits and Expenses		351.4		375.9		1,140.3		1,147.7
Adjusted Operating Income (Loss)		169.4		125.5		428.6		358.6
Depreciation and Amortization		4.5		4.3	[2]	13.3		12.5
Operating Segments | Closed Block
Segment Reporting
Premium Income		192.5		195.0		579.5		629.2
Net Investment Income		245.9		224.7				778.9
Adjusted Net Investment Income						701.1
Other Income		9.6		16.3		30.5		38.3
Segment Adjusted Operating Revenue		448.0		436.0		1,311.1		1,446.4
Policy Benefits		378.8		363.3		1,144.6		1,204.2
Policy Benefits - Remeasurement Loss (Gain)		114.6		659.5		250.0		708.5
Commissions		14.4		16.6		44.1		50.5
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		75.8	[1]	78.9	[2]	228.7	[4]	168.3	[5]
Adjusted Benefits and Expenses		583.6		1,118.3		1,667.4		2,131.5
Adjusted Operating Income (Loss)		(135.6)		(682.3)		(356.3)		(685.1)
Depreciation and Amortization		1.6		1.5	[2]	4.8		4.5
Operating Segments | Corporate
Segment Reporting
Premium Income		0.0		0.0		0.0		0.0
Net Investment Income		23.9		19.7				75.9
Adjusted Net Investment Income	[3]					64.0
Other Income		0.0		0.3		1.1		0.0
Segment Adjusted Operating Revenue		23.9		20.0		65.1		75.9
Policy Benefits		6.4		0.0		8.7		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		54.0		52.2		160.4		156.2
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		9.2	[1]	15.5	[2]	29.8	[3],[4]	40.2	[5]
Adjusted Benefits and Expenses		69.6		67.7		198.9		196.4
Adjusted Operating Income (Loss)		(45.7)		(47.7)		(133.8)		(120.5)
Depreciation and Amortization		$ 2.1		$ 0.1		$ 21.0		$ 0.4

[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 September 30, 2026 was $21.4 million, $6.1 million, $4.5 million, $1.6 million, and $2.1 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 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.

[3] 1Excludes the impact of the strategic actions in the Corporate segment.

[4] 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 nine months ended September 30, 2026 was $64.1 million, $18.1 million, $13.3 million, $4.8 million, and $21.0 million for our Unum US, Unum International, Colonial Life, Closed Block and Corporate segments, respectively.

[5] 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 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.