Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - Interest Rate Swaps - Receive Fixed/Pay Fixed - Designated as Hedging Instrument - Fair Value Hedging - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Fixed Maturity Securities - at fair value (amortized cost of $38,822.6; $38,410.6; allowance for credit losses of $2.9; $2.2)	Fixed Maturity Securities - at fair value (amortized cost of $38,822.6; $38,410.6; allowance for credit losses of $2.9; $2.2)
Hedged Asset, Fair Value Hedge	$ 592.9	$ 529.2
Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge, Cumulative Increase	$ (0.5)	$ (6.1)