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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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 641.0		$ 641.0		$ 714.9
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	1.0	$ 1.0	26.8	$ 1.4
Debt Securities, Available-for-sale, Realized Gain	0.1	$ 0.9	0.3	$ 5.3
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	21.1		21.1
Available-for-sale debt maturities, after 1 but within 5 years, fair value	341.7		341.7
Available-for-sale debt maturities, after 5 but within 10 years, fair value	250.8		250.8
Available-for-sale debt maturities, after 10 years, fair value	19.0		19.0
Debt Securities, Available-for-sale	$ 632.6		$ 632.6