Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - Interest Rate Swaps - Designated as Hedging Instrument - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fixed Maturity Securities | Receive Fixed/Pay Fixed
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 290.6	$ 239.4
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	7.9	1.1
Long-term Debt | Receive Fixed/Pay Variable
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	0.0	(249.2)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 0.0	$ 0.6