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, 2018	Dec. 31, 2017
Fixed Maturity Securities | Receive Variable/Pay Fixed
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 0.0	$ 48.5
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	0.0	0.6
Fixed Maturity Securities | Receive Fixed/Pay Fixed
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	56.2	0.0
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(2.7)	0.0
Long-term Debt | Receive Fixed/Pay Variable
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	(244.4)	(244.8)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 5.1	$ 4.5