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Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 681.2		$ 681.2		$ 683.6
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	7.9	$ 1.8	14.7	$ 15.2
Debt Securities, Available-for-sale, Realized Gain	0.3	$ 4.0	10.9	$ 9.1
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	19.7		19.7
Available-for-sale debt maturities, after 1 but within 5 years, fair value	217.2		217.2
Available-for-sale debt maturities, after 5 but within 10 years, fair value	384.7		384.7
Available-for-sale debt maturities, after 10 years, fair value	57.3		57.3
Debt Securities, Available-for-sale	$ 678.9		$ 678.9