Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	$ 2,560.0
Deferred Policy Acquisition Costs, Amortization Expense	(481.4)	$ (421.1)	$ (452.1)
Deferred Acquisition Costs at End of Year	2,714.5	2,560.0
Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	2,560.0	2,427.4	2,357.7
Deferral of Acquisition Costs	632.2	556.9	523.2
Deferred Policy Acquisition Costs, Amortization Expense	(481.4)	(421.1)	(452.1)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other	3.7	(3.2)	(1.4)
Deferred Acquisition Costs at End of Year	2,714.5	2,560.0	2,427.4
Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,185.1	1,152.9	1,181.0
Deferral of Acquisition Costs	314.7	273.1	257.8
Deferred Policy Acquisition Costs, Amortization Expense	(267.6)	(240.9)	(285.9)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,232.2	1,185.1	1,152.9
Unum US | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(267.6)	(240.9)	(285.9)
Unum International
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	37.0	36.4	32.0
Deferral of Acquisition Costs	14.6	12.0	12.8
Deferred Policy Acquisition Costs, Amortization Expense	(8.4)	(8.2)	(7.0)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other	3.7	(3.2)	(1.4)
Deferred Acquisition Costs at End of Year	46.9	37.0	36.4
Unum International | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(8.4)	(8.2)	(7.0)
Colonial Life
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	1,337.9	1,238.1	1,144.7
Deferral of Acquisition Costs	302.9	271.8	252.6
Deferred Policy Acquisition Costs, Amortization Expense	(205.4)	(172.0)	(159.2)
Deferred Policy Acquisition Cost, Foreign Currency Translation Adjustment and Other	0.0	0.0	0.0
Deferred Acquisition Costs at End of Year	1,435.4	1,337.9	1,238.1
Colonial Life | Operating Segments
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Policy Acquisition Costs, Amortization Expense	(205.4)	(172.0)	(159.2)
Group Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	61.0	60.9	95.3
Deferral of Acquisition Costs	60.2	53.1	49.8
Deferred Policy Acquisition Costs, Amortization Expense	(57.6)	(53.0)	(84.2)
Deferred Acquisition Costs at End of Year	63.6	61.0	60.9
Group Life and AD&D | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	49.3	53.9	76.4
Deferral of Acquisition Costs	38.6	37.3	36.1
Deferred Policy Acquisition Costs, Amortization Expense	(39.0)	(41.9)	(58.6)
Deferred Acquisition Costs at End of Year	48.9	49.3	53.9
Voluntary Benefits | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	601.0	588.6	569.7
Deferral of Acquisition Costs	115.8	100.0	99.4
Deferred Policy Acquisition Costs, Amortization Expense	(106.2)	(87.6)	(80.5)
Deferred Acquisition Costs at End of Year	610.6	601.0	588.6
Individual Disability | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	464.4	441.8	423.6
Deferral of Acquisition Costs	87.4	72.0	63.9
Deferred Policy Acquisition Costs, Amortization Expense	(54.0)	(49.4)	(45.7)
Deferred Acquisition Costs at End of Year	497.8	464.4	441.8
Dental and Vision | Unum US
Movement Analysis of Deferred Policy Acquisition Costs
Deferred Acquisition Costs at Beginning of Year	9.4	7.7	16.0
Deferral of Acquisition Costs	12.7	10.7	8.6
Deferred Policy Acquisition Costs, Amortization Expense	(10.8)	(9.0)	(16.9)
Deferred Acquisition Costs at End of Year	$ 11.3	$ 9.4	$ 7.7