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Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	$ (28.9)	$ (46.9)	$ (3.4)
Debt Securities, Available-for-sale, Realized Gain	9.5	0.3	$ 5.8
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total debt securities	683.6	$ 650.1
Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Due in one year or less	18.6
Due after one year through five years	243.4
Due after five years through ten years	378.7
Due after ten years	41.9
Total debt securities	$ 682.6

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