Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year				$ 2,920.3
Deferred Policy Acquisition Costs, Amortization Expense	$ (144.3)		$ (132.3)	(413.9)	$ (389.9)
Deferred Acquisition Costs at End of Period	3,067.9	$ 2,920.3		3,067.9
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		2,880.4		2,920.3	2,842.8
Deferred Policy Acquisition Cost				566.3	521.0
Deferred Policy Acquisition Costs, Amortization Expense	(144.3)		(132.3)	(413.9)	(389.9)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				(4.8)	6.8
Deferred Acquisition Costs at End of Period	3,067.9	2,920.3	2,880.4	3,067.9	2,880.4
Operating Segments | 2025 Fortitude Re Reinsurance Transaction
Movement Analysis of Deferred Policy Acquisition Costs
Change in Deferred Acquisition Costs					(100.3)
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		1,205.8		1,208.7	1,260.6
Deferred Policy Acquisition Cost				275.5	253.7
Deferred Policy Acquisition Costs, Amortization Expense				(216.8)	(208.2)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				0.0	0.0
Deferred Acquisition Costs at End of Period	1,267.4	1,208.7	1,205.8	1,267.4	1,205.8
Unum US | 2025 Fortitude Re Reinsurance Transaction
Movement Analysis of Deferred Policy Acquisition Costs
Change in Deferred Acquisition Costs					(100.3)
Unum US | Group Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		66.1		63.6	61.1
Deferred Policy Acquisition Cost				51.9	46.5
Deferred Policy Acquisition Costs, Amortization Expense				(44.6)	(41.5)
Deferred Acquisition Costs at End of Period	70.9	63.6	66.1	70.9	66.1
Unum US | Group Life and AD&D
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		59.4		57.6	51.1
Deferred Policy Acquisition Cost				39.0	34.7
Deferred Policy Acquisition Costs, Amortization Expense				(31.9)	(26.4)
Deferred Acquisition Costs at End of Period	64.7	57.6	59.4	64.7	59.4
Unum US | Voluntary Benefits
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		622.2		622.8	614.3
Deferred Policy Acquisition Cost				102.6	93.9
Deferred Policy Acquisition Costs, Amortization Expense				(86.4)	(86.0)
Deferred Acquisition Costs at End of Period	639.0	622.8	622.2	639.0	622.2
Unum US | Individual Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		444.6		451.6	521.2
Deferred Policy Acquisition Cost				71.1	67.2
Deferred Policy Acquisition Costs, Amortization Expense				(42.9)	(43.5)
Deferred Acquisition Costs at End of Period	479.8	451.6	444.6	479.8	444.6
Unum US | Individual Disability | 2025 Fortitude Re Reinsurance Transaction
Movement Analysis of Deferred Policy Acquisition Costs
Change in Deferred Acquisition Costs		(100.3)			(100.3)
Unum US | Dental and Vision
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		13.5		13.1	12.9
Deferred Policy Acquisition Cost				10.9	11.4
Deferred Policy Acquisition Costs, Amortization Expense				(11.0)	(10.8)
Deferred Acquisition Costs at End of Period	13.0	13.1	13.5	13.0	13.5
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(77.2)		(71.4)	(216.8)	(208.2)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		68.4		72.1	53.0
Deferred Policy Acquisition Cost				23.1	16.4
Deferred Policy Acquisition Costs, Amortization Expense				(9.7)	(7.8)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				(4.8)	6.8
Deferred Acquisition Costs at End of Period	80.7	72.1	68.4	80.7	68.4
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(3.6)		(2.7)	(9.7)	(7.8)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		1,606.2		1,639.5	1,529.2
Deferred Policy Acquisition Cost				267.7	250.9
Deferred Policy Acquisition Costs, Amortization Expense				(187.4)	(173.9)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				0.0	0.0
Deferred Acquisition Costs at End of Period	1,719.8	$ 1,639.5	1,606.2	1,719.8	1,606.2
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	$ (63.5)		$ (58.2)	$ (187.4)	$ (173.9)