Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			$ 2,842.8
Deferred Policy Acquisition Costs, Amortization Expense	$ (132.2)	$ (127.9)	(257.6)	$ (254.1)
Deferred Acquisition Costs at End of Period	2,940.3		2,940.3
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			2,842.8	2,714.5
Deferred Policy Acquisition Cost			347.5	332.0
Deferred Policy Acquisition Costs, Amortization Expense	(132.2)	(127.9)	(257.6)	(254.1)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			7.6	(0.9)
Deferred Acquisition Costs at End of Period	2,940.3	2,791.5	2,940.3	2,791.5
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,260.6	1,232.2
Deferred Policy Acquisition Cost			170.4	165.8
Deferred Policy Acquisition Costs, Amortization Expense			(136.8)	(141.0)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	1,294.2	1,257.0	1,294.2	1,257.0
Unum US | Group Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			61.1	63.6
Deferred Policy Acquisition Cost			32.3	32.7
Deferred Policy Acquisition Costs, Amortization Expense			(26.1)	(28.6)
Deferred Acquisition Costs at End of Period	67.3	67.7	67.3	67.7
Unum US | Group Life and AD&D
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			51.1	48.9
Deferred Policy Acquisition Cost			23.7	21.0
Deferred Policy Acquisition Costs, Amortization Expense			(15.7)	(17.1)
Deferred Acquisition Costs at End of Period	59.1	52.8	59.1	52.8
Unum US | Voluntary Benefits
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			614.3	610.6
Deferred Policy Acquisition Cost			63.3	61.1
Deferred Policy Acquisition Costs, Amortization Expense			(57.3)	(58.6)
Deferred Acquisition Costs at End of Period	620.3	613.1	620.3	613.1
Unum US | Individual Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			521.2	497.8
Deferred Policy Acquisition Cost			43.6	43.5
Deferred Policy Acquisition Costs, Amortization Expense			(30.6)	(30.5)
Deferred Acquisition Costs at End of Period	534.2	510.8	534.2	510.8
Unum US | Dental and Vision
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			12.9	11.3
Deferred Policy Acquisition Cost			7.5	7.5
Deferred Policy Acquisition Costs, Amortization Expense			(7.1)	(6.2)
Deferred Acquisition Costs at End of Period	13.3	12.6	13.3	12.6
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(71.6)	(71.2)	(136.8)	(141.0)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			53.0	46.9
Deferred Policy Acquisition Cost			10.9	8.6
Deferred Policy Acquisition Costs, Amortization Expense			(5.1)	(4.8)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			7.6	(0.9)
Deferred Acquisition Costs at End of Period	66.4	49.8	66.4	49.8
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(2.6)	(2.4)	(5.1)	(4.8)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,529.2	1,435.4
Deferred Policy Acquisition Cost			166.2	157.6
Deferred Policy Acquisition Costs, Amortization Expense			(115.7)	(108.3)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	1,579.7	1,484.7	1,579.7	1,484.7
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	$ (58.0)	$ (54.3)	$ (115.7)	$ (108.3)