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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, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	$ (3.4)	$ (1.5)
Debt Securities, Available-for-sale, Realized Gain	5.8	17.7
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Total debt securities	714.9	$ 723.4
Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]
Due in one year or less	26.4
Due after one year through five years	351.9
Due after five years through ten years	293.9
Due after ten years	34.6
Total debt securities	$ 706.8

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