Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 2,714.5
Amortization expense	(521.0)	$ (481.4)	$ (421.1)
Deferred Acquisition Costs at End of Year	2,842.8	2,714.5
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	2,714.5	2,560.0	2,427.4
Capitalization	651.5	632.2	556.9
Amortization expense	(521.0)	(481.4)	(421.1)
Foreign currency	(2.2)	3.7	(3.2)
Deferred Acquisition Costs at End of Year	2,842.8	2,714.5	2,560.0
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,232.2	1,185.1	1,152.9
Capitalization	320.9	314.7	273.1
Amortization expense	(292.5)	(267.6)	(240.9)
Foreign currency	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,260.6	1,232.2	1,185.1
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Capitalization	320.9	314.7	273.1
Amortization expense	(292.5)	(267.6)	(240.9)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	46.9	37.0	36.4
Capitalization	17.8	14.6	12.0
Amortization expense	(9.5)	(8.4)	(8.2)
Foreign currency	(2.2)	3.7	(3.2)
Deferred Acquisition Costs at End of Year	53.0	46.9	37.0
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Capitalization	17.8	14.6	12.0
Amortization expense	(9.5)	(8.4)	(8.2)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,435.4	1,337.9	1,238.1
Capitalization	312.8	302.9	271.8
Amortization expense	(219.0)	(205.4)	(172.0)
Foreign currency	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,529.2	1,435.4	1,337.9
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Capitalization	312.8	302.9	271.8
Amortization expense	(219.0)	(205.4)	(172.0)
Group Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	63.6	61.0	60.9
Capitalization	62.2	60.2	53.1
Amortization expense	(64.7)	(57.6)	(53.0)
Deferred Acquisition Costs at End of Year	61.1	63.6	61.0
Group Life and AD&D | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	48.9	49.3	53.9
Capitalization	40.6	38.6	37.3
Amortization expense	(38.4)	(39.0)	(41.9)
Deferred Acquisition Costs at End of Year	51.1	48.9	49.3
Voluntary Benefits | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	610.6	601.0	588.6
Capitalization	120.2	115.8	100.0
Amortization expense	(116.5)	(106.2)	(87.6)
Deferred Acquisition Costs at End of Year	614.3	610.6	601.0
Individual Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	497.8	464.4	441.8
Capitalization	83.4	87.4	72.0
Amortization expense	(60.0)	(54.0)	(49.4)
Deferred Acquisition Costs at End of Year	521.2	497.8	464.4
Dental and Vision | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	11.3	9.4	7.7
Capitalization	14.5	12.7	10.7
Amortization expense	(12.9)	(10.8)	(9.0)
Deferred Acquisition Costs at End of Year	$ 12.9	$ 11.3	$ 9.4