Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Effects of Reinsurance [Line Items]
Net Realized Investment Gain (Loss)	$ 0.9	$ 33.8	$ 85.5	$ (110.2)
Other Expenses	250.3	256.3	495.3	491.9
Reinsurance Recoverables, Incurred but Not Reported Claims	9,042.2	$ 2,260.2	9,042.2	$ 2,260.2	$ 8,378.9	$ 2,246.8
Closed Block Individual Disability Reinsurance Transaction
Effects of Reinsurance [Line Items]
Amortization of Prepaid Reinsurance Premium	19.7		39.7
Closed Block Individual Disability Reinsurance Transaction | Phase 2
Effects of Reinsurance [Line Items]
Negative Ceding Commission	(18.2)		(18.2)
Net Realized Investment Gain (Loss)			67.6
Increase in Benefits and Change in Reserves for Future Benefits			133.1
Other Expenses			6.2
Reinsurance Recoverables, Incurred but Not Reported Claims	990.0		990.0
Prepaid Reinsurance Premiums	43.1		43.1
Deposit Contracts, Assets	5.0		5.0
Closed Block Individual Disability Reinsurance Transaction | Phases 1 and 2 [Member]
Effects of Reinsurance [Line Items]
Prepaid Reinsurance Premiums	854.8		854.8
Deposit Contracts, Assets	$ 91.8		$ 91.8