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Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - Receive Fixed/Pay Fixed - Designated as Hedging Instrument - Fair Value Hedging - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Derivative Instruments and Hedging Activities Disclosures |  |
Hedged Asset, Fair Value Hedge | $ 690.4 | $ 551.0
Hedged Asset, Statement of Financial Position [Extensible Enumeration] | Debt Securities, Available-for-Sale | Debt Securities, Available-for-Sale
Fixed Maturity Securities |  |
Derivative Instruments and Hedging Activities Disclosures |  |
Hedged Asset, Fair Value Hedge, Cumulative Increase | $ 23.5 | $ (46.3)
hedged asset, amortized cost | $ 792.3 | $ 648.4

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