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	Dec. 31, 2019	Dec. 31, 2018
Fixed Maturity Securities | Receive Fixed/Pay Fixed
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 239.4	$ 56.2
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	1.1	(2.7)
Long-term Debt | Receive Fixed/Pay Variable
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	(249.2)	(244.4)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 0.6	$ 5.1