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Fair Value (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Fair Value Disclosure [Line Items] |  |
Transfers between Fair Value Hierarchies | $ 0.0 | $ 0.0
Allowance for Credit Loss | 0.4 | 0.6
U.S. Treasury debt securities |  |
Fair Value Disclosure [Line Items] |  |
Allowance for Credit Loss | 0.0 | 0.0
Corporate/Other debt securities |  |
Fair Value Disclosure [Line Items] |  |
Allowance for Credit Loss | $ 0.4 | $ 0.6

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