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Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - Debt securities - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 1.5	$ 5.7	$ 4.4	$ 9.4
Gross realized losses	0.1	$ 0.7	0.4	$ 0.9
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity
Available-for-sale debt maturities due within one year, fair value	30.1		30.1
Available-for-sale debt maturities, after 1 but within 5 years, fair value	308.6		308.6
Available-for-sale debt maturities, after 5 but within 10 years, fair value	331.3		331.3
Available-for-sale debt maturities, after 10 years, fair value	40.7		40.7
Debt Securities, Available-for-sale	$ 710.7		$ 710.7

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- Definition

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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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 second through fifth 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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