Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			$ 2,714.5
Deferred Policy Acquisition Costs, Amortization Expense	$ (127.9)	$ (113.7)	(254.1)	$ (229.6)
Deferred Acquisition Costs at End of Period	2,791.5		2,791.5
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			2,714.5	2,560.0
Deferred Policy Acquisition Cost			332.0	314.2
Deferred Policy Acquisition Costs, Amortization Expense			(254.1)	(229.6)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			(0.9)	2.6
Deferred Acquisition Costs at End of Period	2,791.5	2,647.2	2,791.5	2,647.2
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,232.2	1,185.1
Deferred Policy Acquisition Cost			165.8	156.1
Deferred Policy Acquisition Costs, Amortization Expense			(141.0)	(129.2)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	1,257.0	1,212.0	1,257.0	1,212.0
Unum US | Group Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			63.6	61.0
Deferred Policy Acquisition Cost			32.7	30.4
Deferred Policy Acquisition Costs, Amortization Expense			(28.6)	(25.9)
Deferred Acquisition Costs at End of Period	67.7	65.5	67.7	65.5
Unum US | Group Life and AD&D
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			48.9	49.3
Deferred Policy Acquisition Cost			21.0	19.9
Deferred Policy Acquisition Costs, Amortization Expense			(17.1)	(19.3)
Deferred Acquisition Costs at End of Period	52.8	49.9	52.8	49.9
Unum US | Voluntary Benefits
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			610.6	601.0
Deferred Policy Acquisition Cost			61.1	57.6
Deferred Policy Acquisition Costs, Amortization Expense			(58.6)	(52.2)
Deferred Acquisition Costs at End of Period	613.1	606.4	613.1	606.4
Unum US | Individual Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			497.8	464.4
Deferred Policy Acquisition Cost			43.5	42.1
Deferred Policy Acquisition Costs, Amortization Expense			(30.5)	(26.7)
Deferred Acquisition Costs at End of Period	510.8	479.8	510.8	479.8
Unum US | Dental and Vision
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			11.3	9.4
Deferred Policy Acquisition Cost			7.5	6.1
Deferred Policy Acquisition Costs, Amortization Expense			(6.2)	(5.1)
Deferred Acquisition Costs at End of Period	12.6	10.4	12.6	10.4
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			46.9	37.0
Deferred Policy Acquisition Cost			8.6	7.0
Deferred Policy Acquisition Costs, Amortization Expense			(4.8)	(3.7)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			(0.9)	2.6
Deferred Acquisition Costs at End of Period	49.8	42.9	49.8	42.9
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,435.4	1,337.9
Deferred Policy Acquisition Cost			157.6	151.1
Deferred Policy Acquisition Costs, Amortization Expense			(108.3)	(96.7)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	$ 1,484.7	$ 1,392.3	$ 1,484.7	$ 1,392.3