v3.25.2
Marketable Securities Held in Trusts - Maturity Dates and Realized Gain and Loss (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity          
Debt Securities, Available-for-sale $ 722.6   $ 722.6   $ 695.2
Debt securities          
Debt Securities, Available-for-sale [Line Items]          
Debt Securities, Available-for-sale, Realized Loss 7.2 $ 7.9 7.8 $ 14.7  
Debt Securities, Available-for-sale, Realized Gain 0.3 $ 0.3 0.7 $ 10.9  
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity          
Available-for-sale debt maturities due within one year, fair value 19.9   19.9    
Available-for-sale debt maturities, after 1 but within 5 years, fair value 239.8   239.8    
Available-for-sale debt maturities, after 5 but within 10 years, fair value 413.2   413.2    
Available-for-sale debt maturities, after 10 years, fair value 51.8   51.8    
Debt Securities, Available-for-sale $ 724.7   $ 724.7