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Marketable Securities Held in Trusts (Tables)
 12 Months Ended
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Debt Securities, Available-for-sale
The estimated fair value of the investments in the RCRA Trusts as of December 31, 2024 and December 31, 2023 are as follows:
December 31, 2024
(in millions)Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
Level 1
    Cash and cash equivalents $3.1 $— $— $3.1 
Level 2
    Corporate debt securities203.3 0.8 (6.8)197.3 
    Municipal bonds210.8 0.7 (4.0)207.5 
    U.S. government bonds295.1 — (7.8)287.3 
Total$712.3 $1.5 $(18.6)$695.2 
December 31, 2023
(in millions)Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
Level 1
    Cash and cash equivalents $1.0 $— $— $1.0 
Level 2
    Corporate debt securities204.6 1.9 (8.4)198.1 
    Municipal bonds206.9 1.9 (4.1)204.7 
    U.S. government bonds268.6 11.5 (0.3)279.8 
Total$681.1 $15.3 $(12.8)$683.6 
Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value
The following tables show gross unrealized losses and fair values of the RCRA Trusts’ available-for-sale securities that have been in a continuous unrealized loss position for which an allowance for credit losses has not been recorded as of December 31, 2024 and December 31, 2023.
December 31, 2024December 31, 2023
Securities that have been in a continuous loss position for less than 12 months (in millions):
Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Corporate debt securities$53.4 $(0.7)$5.4 $(0.1)
Municipal bonds102.4 (1.7)42.3 (0.2)
U.S. government bonds280.9 (7.8)26.4 (0.3)
Total$436.7 $(10.2)$74.1 $(0.6)
December 31, 2024December 31, 2023
Securities that have been in a continuous loss position for more than 12 months (in millions):
Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Corporate debt securities$81.3 $(6.1)$121.5 $(8.3)
Municipal bonds55.6 (2.3)84.1 (3.9)
U.S. government bonds— — — — 
Total$136.9 $(8.4)$205.6 $(12.2)
Schedule of maturity dates for debt securities
The following table summarizes the balance by contractual maturity of the available-for-sale debt securities invested by the RCRA Trusts as of December 31, 2024. Actual maturities may differ from contractual maturities because the issuers of the securities may have the right to prepay obligations before the underlying contracts mature.
(in millions)December 31, 2024
Due in one year or less$21.3 
Due after one year through five years218.9 
Due after five years through ten years402.1 
Due after ten years49.8 
Total debt securities$692.1