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Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 713.2		$ 713.2		$ 683.6
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	1.2	$ 5.7	10.4	$ 20.9
Debt Securities, Available-for-sale, Realized Gain	6.4	$ 0.1	17.2	$ 9.2
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	24.1		24.1
Available-for-sale debt maturities, after 1 but within 5 years, fair value	275.3		275.3
Available-for-sale debt maturities, after 5 but within 10 years, fair value	350.4		350.4
Available-for-sale debt maturities, after 10 years, fair value	61.5		61.5
Debt Securities, Available-for-sale	$ 711.3		$ 711.3

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