Liability for Unpaid Claims and CAE - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Liability for Unpaid Claims and CAE
Unum US Group Long-term Disability Reserve Reduction, Before Tax	$ (215.0)
Closed Block Long-term Care Reserve Increase	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)
Reinsurance Recoverables, Incurred but Not Reported Claims	8,854.8	$ 8,378.9	$ 2,261.7	$ 2,246.8
Phase 2 | Closed Block Individual Disability Reinsurance Transaction
Liability for Unpaid Claims and CAE
Increase in Benefits and Change in Reserves for Future Benefits	133.1
Reinsurance Recoverables, Incurred but Not Reported Claims	$ 990.0