Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 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	$ (133.8)	$ (129.1)	(387.9)	$ (358.7)
Deferred Acquisition Costs at End of Period	2,823.5		2,823.5
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			495.3	467.5
Deferred Policy Acquisition Costs, Amortization Expense			(387.9)	(358.7)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			1.6	(0.1)
Deferred Acquisition Costs at End of Period	2,823.5	2,668.7	2,823.5	2,668.7
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			247.4	230.6
Deferred Policy Acquisition Costs, Amortization Expense			(216.7)	(199.2)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	1,262.9	1,216.5	1,262.9	1,216.5
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			48.1	44.7
Deferred Policy Acquisition Costs, Amortization Expense			(46.8)	(42.1)
Deferred Acquisition Costs at End of Period	64.9	63.6	64.9	63.6
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			31.0	28.9
Deferred Policy Acquisition Costs, Amortization Expense			(26.7)	(29.5)
Deferred Acquisition Costs at End of Period	53.2	48.7	53.2	48.7
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			93.2	85.2
Deferred Policy Acquisition Costs, Amortization Expense			(88.4)	(79.5)
Deferred Acquisition Costs at End of Period	615.4	606.7	615.4	606.7
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			63.9	62.6
Deferred Policy Acquisition Costs, Amortization Expense			(45.3)	(40.2)
Deferred Acquisition Costs at End of Period	516.4	486.8	516.4	486.8
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			11.2	9.2
Deferred Policy Acquisition Costs, Amortization Expense			(9.5)	(7.9)
Deferred Acquisition Costs at End of Period	13.0	10.7	13.0	10.7
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			46.9	37.0
Deferred Policy Acquisition Cost			13.2	10.8
Deferred Policy Acquisition Costs, Amortization Expense			(7.3)	(6.1)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			1.6	(0.1)
Deferred Acquisition Costs at End of Period	54.4	41.6	54.4	41.6
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			234.7	226.1
Deferred Policy Acquisition Costs, Amortization Expense			(163.9)	(153.4)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	$ 1,506.2	$ 1,410.6	$ 1,506.2	$ 1,410.6