Liability for Unpaid Claims - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Mar. 31, 2021	Dec. 16, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Unpaid Claims and CAE Narrative Information
Unum US Group Long-term Disability Reserve Reduction, Before Tax	$ 215.0
Closed Block Long-term Care Reserve Increase, Before Tax	2.1
Closed Block Individual Disability Reserve Increase, Before Tax	6.4
Net Reduction in Unpaid Claims as a Result of Assumption Updates, Before Tax	(206.5)
Long-term Care Reserve Increase, Before Tax		$ 151.5
Reinsurance Recoverables, Incurred but Not Reported Claims	8,697.8	8,378.9			$ 2,246.8	$ 2,227.3
Closed Block Individual Disability Reinsurance Transaction
Liability for Unpaid Claims and CAE Narrative Information
Increase in Benefits and Change in Reserves for Future Benefits	133.1	1,284.5
Reinsurance Recoverables, Incurred but Not Reported Claims			$ 990.0	$ 6,141.5
Closed Block Individual Disability Reinsurance Transaction | Phase 1
Liability for Unpaid Claims and CAE Narrative Information
Increase in Benefits and Change in Reserves for Future Benefits		1,284.5
Reinsurance Recoverables, Incurred but Not Reported Claims		6,141.5
Closed Block Individual Disability Reinsurance Transaction | Phase 2
Liability for Unpaid Claims and CAE Narrative Information
Increase in Benefits and Change in Reserves for Future Benefits	133.1
Reinsurance Recoverables, Incurred but Not Reported Claims	$ 990.0
Claim Reserve
Liability for Unpaid Claims and CAE Narrative Information
Long-term Care Reserve Increase, Before Tax		$ 7.0