Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information
Premium Income	$ 9,623.4	$ 9,481.0	$ 9,378.1
Net Investment Income	2,122.2	2,213.2	2,360.7
Other Income	261.1	242.9	224.2
Interest and Debt Expense	188.5	185.0	188.2
Operating Segments
Segment Reporting Information
Premium Income	9,623.4	9,481.0	9,378.1
Net Investment Income	2,122.2	2,213.2	2,360.7
Other Income	261.1	242.9	224.2
Adjusted Operating Revenue	12,006.7	11,937.1	11,963.0
Adjusted Operating Income (Loss)	1,555.9	1,116.6	1,278.0
Interest and Debt Expense	192.7	185.0	188.2
Depreciation and Amortization	701.4	703.1	722.3
Operating Segments \| Unum US
Segment Reporting Information
Premium Income	6,258.3	6,078.0	6,018.9
Other Income	196.3	170.0	154.9
Adjusted Operating Revenue	7,130.9	6,969.6	6,894.1
Adjusted Operating Income (Loss)	970.7	464.9	825.4
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	370.6	400.9	421.7
Operating Segments \| Unum International
Segment Reporting Information
Premium Income	718.8	717.0	652.8
Net Investment Income	170.1
Other Income	0.9	0.6	0.5
Adjusted Operating Revenue	889.8	850.3	757.9
Adjusted Operating Income (Loss)	127.0	105.7	76.6
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	22.0	21.0	20.1
Operating Segments \| Colonial Life
Segment Reporting Information
Premium Income	1,702.0	1,690.2	1,712.0
Other Income	1.1	1.0	1.1
Adjusted Operating Revenue	1,855.8	1,863.2	1,868.8
Adjusted Operating Income (Loss)	374.6	329.2	335.4
Interest and Debt Expense	0.0	0.0	0.0
Depreciation and Amortization	303.5	275.4	273.9
Operating Segments \| Closed Block
Segment Reporting Information
Premium Income	944.3	995.8	994.4
Other Income	58.0	65.1	66.6
Adjusted Operating Revenue	2,072.9	2,219.9	2,431.3
Adjusted Operating Income (Loss)	247.9	394.7	241.4
Interest and Debt Expense	0.0	0.0	3.1
Depreciation and Amortization	4.7	5.1	5.9
Operating Segments \| Corporate and Other Segment
Segment Reporting Information
Premium Income	0.0	0.0	0.0
Other Income	4.8	6.2	1.1
Adjusted Operating Revenue	57.3	34.1	10.9
Adjusted Operating Income (Loss)	(164.3)	(177.9)	(200.8)
Interest and Debt Expense	192.7	185.0	185.1
Depreciation and Amortization	$ 0.6	$ 0.7	$ 0.7

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