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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, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Debt Securities, Available-for-sale [Line Items]        
Available-for-sale Securities, Gross Realized Gains $ 5.7   $ 9.4 $ 3.1
Gross realized losses 0.7 $ 0.4 0.9 $ 1.2
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity        
Available-for-sale debt maturities due within one year, fair value 26.9   26.9  
Available-for-sale debt maturities, after 1 but within 5 years, fair value 239.7   239.7  
Available-for-sale debt maturities, after 5 but within 10 years, fair value 401.9   401.9  
Available-for-sale debt maturities, after 10 years, fair value 38.7   38.7  
Debt Securities, Available-for-sale $ 707.2   $ 707.2