Restricted Securities Available for Sale (Schedule of Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Within one year, cost	$ 7.9	$ 4.0
Over one year to five years, cost	74.4	66.4
Over five years to ten years, cost	7.7	5.6
Over ten years, cost	0.0	0.1
Total restricted securities available for sale, cost	90.0	76.1
Within one year, fair value	7.7	3.9
Over one year to five years, fair value	71.0	63.0
Over five years to ten years, fair value	7.5	5.3
Over ten years, fair value	0.0	0.1
Debt Securities, Available-for-sale, Restricted	$ 86.2	$ 72.3

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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