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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, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	$ (15.1)	$ (28.9)	$ (46.9)
Debt Securities, Available-for-sale, Realized Gain	17.5	9.5	$ 0.3
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total debt securities	695.2	$ 683.6
Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Due in one year or less	21.3
Due after one year through five years	218.9
Due after five years through ten years	402.1
Due after ten years	49.8
Total debt securities	$ 692.1

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