Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025	Jun. 30, 2026	Jun. 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	$ (135.4)		$ (132.2)	(269.6)	$ (257.6)
Deferred Acquisition Costs at End of Period	3,025.4			3,025.4
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		$ 2,940.3		2,920.3	2,842.8
Deferred Policy Acquisition Cost				378.0	347.5
Deferred Policy Acquisition Costs, Amortization Expense	(135.4)		(132.2)	(269.6)	(257.6)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				(3.3)	7.6
Deferred Acquisition Costs at End of Period	3,025.4		2,940.3	3,025.4	2,940.3
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		1,294.2		1,208.7	1,260.6
Deferred Policy Acquisition Cost				181.3	170.4
Deferred Policy Acquisition Costs, Amortization Expense				(139.6)	(136.8)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				0.0	0.0
Deferred Acquisition Costs at End of Period	1,250.4		1,294.2	1,250.4	1,294.2
Unum US | Group Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		67.3		63.6	61.1
Deferred Policy Acquisition Cost				34.0	32.3
Deferred Policy Acquisition Costs, Amortization Expense				(28.3)	(26.1)
Deferred Acquisition Costs at End of Period	69.3		67.3	69.3	67.3
Unum US | Group Life and AD&D
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		59.1		57.6	51.1
Deferred Policy Acquisition Cost				25.7	23.7
Deferred Policy Acquisition Costs, Amortization Expense				(20.0)	(15.7)
Deferred Acquisition Costs at End of Period	63.3		59.1	63.3	59.1
Unum US | Voluntary Benefits
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		620.3		622.8	614.3
Deferred Policy Acquisition Cost				66.8	63.3
Deferred Policy Acquisition Costs, Amortization Expense				(56.6)	(57.3)
Deferred Acquisition Costs at End of Period	633.0		620.3	633.0	620.3
Unum US | Individual Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		534.2		451.6	521.2
Deferred Policy Acquisition Cost				47.6	43.6
Deferred Policy Acquisition Costs, Amortization Expense				(27.4)	(30.6)
Deferred Acquisition Costs at End of Period	471.8		534.2	471.8	534.2
Unum US | Individual Disability | 2025 Fortitude Re Reinsurance Transaction
Movement Analysis of Deferred Policy Acquisition Costs
Change in Deferred Acquisition Costs		(100.3)
Unum US | Dental and Vision
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		13.3		13.1	12.9
Deferred Policy Acquisition Cost				7.2	7.5
Deferred Policy Acquisition Costs, Amortization Expense				(7.3)	(7.1)
Deferred Acquisition Costs at End of Period	13.0		13.3	13.0	13.3
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(70.9)		(71.6)	(139.6)	(136.8)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		66.4		72.1	53.0
Deferred Policy Acquisition Cost				16.3	10.9
Deferred Policy Acquisition Costs, Amortization Expense				(6.1)	(5.1)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				(3.3)	7.6
Deferred Acquisition Costs at End of Period	79.0		66.4	79.0	66.4
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(2.7)		(2.6)	(6.1)	(5.1)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year		$ 1,579.7		1,639.5	1,529.2
Deferred Policy Acquisition Cost				180.4	166.2
Deferred Policy Acquisition Costs, Amortization Expense				(123.9)	(115.7)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other				0.0	0.0
Deferred Acquisition Costs at End of Period	1,696.0		1,579.7	1,696.0	1,579.7
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	$ (61.8)		$ (58.0)	$ (123.9)	$ (115.7)