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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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 734.2		$ 734.2		$ 695.2
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	0.8	$ 1.2	8.6	$ 10.4
Debt Securities, Available-for-sale, Realized Gain	0.6	$ 6.4	1.3	$ 17.2
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	9.8		9.8
Available-for-sale debt maturities, after 1 but within 5 years, fair value	242.3		242.3
Available-for-sale debt maturities, after 5 but within 10 years, fair value	425.7		425.7
Available-for-sale debt maturities, after 10 years, fair value	53.9		53.9
Debt Securities, Available-for-sale	$ 731.7		$ 731.7