Liability for Unpaid Claims - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 16, 2020	Dec. 31, 2017
Liability for Unpaid Claims and CAE Narrative Information
Long-term Care Reserve Increase, Before Tax	$ 151.5		$ 750.8
Reinsurance Recoverables, Incurred but Not Reported Claims	8,378.9	$ 2,246.8	2,227.3		$ 2,182.0
Benefits and Change in Reserves for Future Benefits	8,972.9	7,496.2	8,020.4
Closed Block Individual Disability Reinsurance Transaction
Liability for Unpaid Claims and CAE Narrative Information
Reinsurance Recoverables, Incurred but Not Reported Claims	6,141.5			$ 6,141.5
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain (Loss) on Securities
Liability for Unpaid Claims and CAE Narrative Information
Benefits and Change in Reserves for Future Benefits	1,284.5	$ 0.0	0.0
Claim Reserve
Liability for Unpaid Claims and CAE Narrative Information
Long-term Care Reserve Increase, Before Tax	$ 7.0		$ 236.0