Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Within one year, cost	$ 4.8	$ 6.9
Over one year to five years, cost	87.6	80.5
Over five years to ten years, cost	9.0	7.1
Over ten years, cost	0.1	0.1
Total restricted securities available for sale, cost	101.5	94.6
Within one year, fair value	4.7	6.8
Over one year to five years, fair value	86.1	79.1
Over five years to ten years, fair value	9.0	7.2
Over ten years, fair value	0.1	0.1
Debt Securities, Available-for-sale, Restricted	$ 99.9	$ 93.2

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Topic 944
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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