Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	$ 2,842.8
Amortization expense	(125.4)	$ (126.2)
Deferred Acquisition Costs at End of Period	2,893.2
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	2,842.8	2,714.5
Capitalization	172.6	166.9
Amortization expense	(125.4)	(126.2)
Foreign currency	3.2	(0.5)
Deferred Acquisition Costs at End of Period	2,893.2	2,754.7
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,260.6	1,232.2
Capitalization	84.7	83.3
Amortization expense	(65.2)	(69.8)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,280.1	1,245.7
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(65.2)	(69.8)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	53.0	46.9
Capitalization	5.3	4.3
Amortization expense	(2.5)	(2.4)
Foreign currency	3.2	(0.5)
Deferred Acquisition Costs at End of Period	59.0	48.3
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(2.5)	(2.4)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	1,529.2	1,435.4
Capitalization	82.6	79.3
Amortization expense	(57.7)	(54.0)
Foreign currency	0.0	0.0
Deferred Acquisition Costs at End of Period	1,554.1	1,460.7
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Amortization expense	(57.7)	(54.0)
Group Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	61.1	63.6
Capitalization	16.1	16.5
Amortization expense	(10.4)	(14.4)
Deferred Acquisition Costs at End of Period	66.8	65.7
Group Life and AD&D | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	51.1	48.9
Capitalization	11.6	10.5
Amortization expense	(6.1)	(6.9)
Deferred Acquisition Costs at End of Period	56.6	52.5
Voluntary Benefits | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	614.3	610.6
Capitalization	32.2	30.5
Amortization expense	(29.8)	(29.4)
Deferred Acquisition Costs at End of Period	616.7	611.7
Individual Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	521.2	497.8
Capitalization	21.0	22.0
Amortization expense	(15.4)	(16.1)
Deferred Acquisition Costs at End of Period	526.8	503.7
Dental and Vision | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Period	12.9	11.3
Capitalization	3.8	3.8
Amortization expense	(3.5)	(3.0)
Deferred Acquisition Costs at End of Period	$ 13.2	$ 12.1