Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	$ 2,920.3
Amortization expense	(134.2)	$ (125.4)
Deferred Acquisition Costs at End of Period	2,974.4
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	2,920.3	2,842.8
Capitalization	190.6	172.6
Amortization expense	(134.2)	(125.4)
Foreign currency	(2.3)	3.2
Deferred Acquisition Costs at End of Period	2,974.4	2,893.2
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,208.7	1,260.6
Capitalization	91.6	84.7
Amortization expense	(68.7)	(65.2)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,231.6	1,280.1
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(68.7)	(65.2)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	72.1	53.0
Capitalization	9.2	5.3
Amortization expense	(3.4)	(2.5)
Foreign currency	(2.3)	3.2
Deferred Acquisition Costs at End of Period	75.6	59.0
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(3.4)	(2.5)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,639.5	1,529.2
Capitalization	89.8	82.6
Amortization expense	(62.1)	(57.7)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,667.2	1,554.1
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(62.1)	(57.7)
Group Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	63.6	61.1
Capitalization	16.3	16.1
Amortization expense	(13.9)	(10.4)
Deferred Acquisition Costs at End of Period	66.0	66.8
Group Life and AD&D | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	57.6	51.1
Capitalization	12.3	11.6
Amortization expense	(8.7)	(6.1)
Deferred Acquisition Costs at End of Period	61.2	56.6
Voluntary Benefits | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	622.8	614.3
Capitalization	34.0	32.2
Amortization expense	(28.8)	(29.8)
Deferred Acquisition Costs at End of Period	628.0	616.7
Individual Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	451.6	521.2
Capitalization	25.4	21.0
Amortization expense	(13.6)	(15.4)
Deferred Acquisition Costs at End of Period	463.4	526.8
Dental and Vision | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	13.1	12.9
Capitalization	3.6	3.8
Amortization expense	(3.7)	(3.5)
Deferred Acquisition Costs at End of Period	$ 13.0	$ 13.2