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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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Debt Securities, Available-for-sale	$ 718.2		$ 718.2		$ 723.4
Debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Realized Loss	1.0	$ 0.4	1.4	$ 1.1
Debt Securities, Available-for-sale, Realized Gain	0.9	$ 5.7	5.3	$ 14.8
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	22.9		22.9
Available-for-sale debt maturities, after 1 but within 5 years, fair value	414.8		414.8
Available-for-sale debt maturities, after 5 but within 10 years, fair value	230.3		230.3
Available-for-sale debt maturities, after 10 years, fair value	43.3		43.3
Debt Securities, Available-for-sale	$ 711.3		$ 711.3

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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