Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 682.7		$ 683.6
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	6.8	$ 13.4
Debt Securities, Available-for-sale, Realized Gain	10.6	$ 5.1
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	19.8
Available-for-sale debt maturities, after 1 but within 5 years, fair value	234.2
Available-for-sale debt maturities, after 5 but within 10 years, fair value	376.3
Available-for-sale debt maturities, after 10 years, fair value	47.0
Debt Securities, Available-for-sale	$ 677.3

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