Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			$ 2,560.0
Deferred Policy Acquisition Costs, Amortization Expense	$ (113.7)	$ (106.5)	(229.6)	$ (210.1)
Deferred Acquisition Costs at End of Period	2,647.2	2,493.3	2,647.2	2,493.3
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			2,560.0	2,427.4
Deferred Policy Acquisition Cost			314.2	279.7
Deferred Policy Acquisition Costs, Amortization Expense			(229.6)	(210.1)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			2.6	(3.7)
Deferred Acquisition Costs at End of Period	2,647.2	2,493.3	2,647.2	2,493.3
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,185.1	1,152.9
Deferred Policy Acquisition Cost			156.1	133.2
Deferred Policy Acquisition Costs, Amortization Expense			(129.2)	(115.6)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	1,212.0	1,170.5	1,212.0	1,170.5
Unum US | Group Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			61.0	60.9
Deferred Policy Acquisition Cost			30.4	25.2
Deferred Policy Acquisition Costs, Amortization Expense			(25.9)	(23.8)
Deferred Acquisition Costs at End of Period	65.5	62.3	65.5	62.3
Unum US | Group Life and AD&D
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			49.3	53.9
Deferred Policy Acquisition Cost			19.9	17.9
Deferred Policy Acquisition Costs, Amortization Expense			(19.3)	(19.5)
Deferred Acquisition Costs at End of Period	49.9	52.3	49.9	52.3
Unum US | Individual Disability
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			464.4	441.8
Deferred Policy Acquisition Cost			42.1	33.0
Deferred Policy Acquisition Costs, Amortization Expense			(26.7)	(24.3)
Deferred Acquisition Costs at End of Period	479.8	450.5	479.8	450.5
Unum US | Voluntary Benefits
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			601.0	588.6
Deferred Policy Acquisition Cost			57.6	52.2
Deferred Policy Acquisition Costs, Amortization Expense			(52.2)	(43.6)
Deferred Acquisition Costs at End of Period	606.4	597.2	606.4	597.2
Unum US | Dental and Vision
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			9.4	7.7
Deferred Policy Acquisition Cost			6.1	4.9
Deferred Policy Acquisition Costs, Amortization Expense			(5.1)	(4.4)
Deferred Acquisition Costs at End of Period	10.4	8.2	10.4	8.2
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			37.0	36.4
Deferred Policy Acquisition Cost			7.0	6.4
Deferred Policy Acquisition Costs, Amortization Expense			(3.7)	(4.8)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			2.6	(3.7)
Deferred Acquisition Costs at End of Period	42.9	34.3	42.9	34.3
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year			1,337.9	1,238.1
Deferred Policy Acquisition Cost			151.1	140.1
Deferred Policy Acquisition Costs, Amortization Expense			(96.7)	(89.7)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other			0.0	0.0
Deferred Acquisition Costs at End of Period	$ 1,392.3	$ 1,288.5	$ 1,392.3	$ 1,288.5