Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 709.5		$ 695.2
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	0.5	$ 6.8
Debt Securities, Available-for-sale, Realized Gain	0.4	$ 10.6
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	18.9
Available-for-sale debt maturities, after 1 but within 5 years, fair value	225.1
Available-for-sale debt maturities, after 5 but within 10 years, fair value	414.3
Available-for-sale debt maturities, after 10 years, fair value	48.9
Debt Securities, Available-for-sale	$ 707.2

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