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Marketable Securities Held in Trusts (Tables)
 9 Months Ended
Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract]  
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 September 30, 2025. 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.
September 30, 2025
Due in one year or less$9.8 
Due after one year through five years242.3 
Due after five years through ten years425.7 
Due after ten years53.9 
Total debt securities$731.7 
Unrealized Gain (Loss) on Investments
The estimated fair value of the investments in the RCRA Trusts as of September 30, 2025 and December 31, 2024 are as follows:
September 30, 2025
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
Level 1
    Cash and cash equivalents $2.5 $— $— $2.5 
Level 2
    Corporate debt securities218.1 3.9 (3.1)218.9 
    Municipal bonds212.0 3.4 (1.5)213.9 
    U.S. government bonds291.0 7.9 — 298.9 
Total$723.6 $15.2 $(4.6)$734.2 
December 31, 2024
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 
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 September 30, 2025 and December 31, 2024:
September 30, 2025December 31, 2024
(in millions)Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Securities that have been in a continuous loss position for less than 12 months:
Corporate debt securities$8.7 $— $53.4 $(0.7)
Municipal bonds14.3 (0.2)102.4 (1.7)
U.S. government bonds9.4 — 280.9 (7.8)
$32.4 $(0.2)$436.7 $(10.2)
Securities that have been in a continuous loss position for more than 12 months:
Corporate debt securities$55.2 $(3.1)$81.3 $(6.1)
Municipal bonds48.4 (1.3)55.6 (2.3)
$103.6 $(4.4)$136.9 $(8.4)