Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	$ 2,714.5
Amortization expense	(126.2)	$ (115.9)
Deferred Acquisition Costs at End of Period	2,754.7
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	2,714.5	2,560.0
Capitalization	166.9	157.7
Amortization expense	(126.2)	(115.9)
Foreign currency	(0.5)	0.6
Deferred Acquisition Costs at End of Period	2,754.7	2,602.4
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,232.2	1,185.1
Capitalization	83.3	79.0
Amortization expense	(69.8)	(65.8)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,245.7	1,198.3
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	46.9	37.0
Capitalization	4.3	3.5
Amortization expense	(2.4)	(1.8)
Foreign currency	(0.5)	0.6
Deferred Acquisition Costs at End of Period	48.3	39.3
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,435.4	1,337.9
Capitalization	79.3	75.2
Amortization expense	(54.0)	(48.3)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,460.7	1,364.8
Group Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	63.6	61.0
Capitalization	16.5	14.6
Amortization expense	(14.4)	(13.1)
Deferred Acquisition Costs at End of Period	65.7	62.5
Group Life and AD&D | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	48.9	49.3
Capitalization	10.5	9.8
Amortization expense	(6.9)	(9.8)
Deferred Acquisition Costs at End of Period	52.5	49.3
Voluntary Benefits | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	610.6	601.0
Capitalization	30.5	30.3
Amortization expense	(29.4)	(27.1)
Deferred Acquisition Costs at End of Period	611.7	604.2
Individual Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	497.8	464.4
Capitalization	22.0	21.3
Amortization expense	(16.1)	(13.3)
Deferred Acquisition Costs at End of Period	503.7	472.4
Dental and Vision | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	11.3	9.4
Capitalization	3.8	3.0
Amortization expense	(3.0)	(2.5)
Deferred Acquisition Costs at End of Period	$ 12.1	$ 9.9