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, 2018	Dec. 31, 2017
Receive Variable/Pay Fixed | Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 48.1	$ 48.5
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	0.1	0.6
Receive Fixed/Pay Fixed | Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	19.4	0.0
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(0.3)	0.0
Receive Fixed/Pay Variable | Long-term Debt
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	(242.5)	(244.8)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 6.9	$ 4.5