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	Mar. 31, 2019	Dec. 31, 2018
Receive Fixed/Pay Fixed | Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 88.2	$ 56.2
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(3.0)	(2.7)
Receive Fixed/Pay Variable | Long-term Debt
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	(245.9)	(244.4)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 3.7	$ 5.1