Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Fixed Maturity Securities - at fair value (amortized cost of $38,410.6; $37,825.2; allowance for credit losses of $2.2; $—)	Fixed Maturity Securities - at fair value (amortized cost of $38,410.6; $37,825.2; allowance for credit losses of $2.2; $—)
Interest Rate Swaps \| Receive Fixed/Pay Fixed \| Designated as Hedging Instrument \| Fair Value Hedging
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 529.2	$ 394.4
Interest Rate Swaps \| Receive Fixed/Pay Fixed \| Designated as Hedging Instrument \| Fair Value Hedging \| Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	$ (6.1)	$ (24.8)

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Amount of asset hedged in fair value hedging relationship.

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

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Indicates line item in statement of financial position that includes hedged asset.

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