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Marketable Securities Held in Trusts (Tables)
 12 Months Ended
Dec. 31, 2023
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, 2023 and December 31, 2022 are as follows:
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 
December 31, 2022
(in millions)Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
Level 1
    Cash and cash equivalents $7.7 $— $— $7.7 
Level 2
    Corporate debt securities203.8 0.1 (17.1)186.8 
    Municipal bonds197.0 0.4 (8.0)189.4 
    U.S. government bonds269.6 — (3.6)266.0 
    Other holdings0.2 — — 0.2 
Total$678.3 $0.5 $(28.7)$650.1 
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, 2023 and December 31, 2022.
December 31, 2023December 31, 2022
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$5.4 $(0.1)$105.6 $(6.5)
Municipal bonds42.3 (0.2)104.7 (2.9)
U.S. government bonds26.4 (0.3)264.9 (3.5)
Total$74.1 $(0.6)$475.2 $(12.9)
December 31, 2023December 31, 2022
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$121.5 $(8.3)$72.8 $(10.6)
Municipal bonds84.1 (3.9)61.9 (5.1)
U.S. government bonds— — 0.8 (0.1)
Total$205.6 $(12.2)$135.5 $(15.8)
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, 2023. 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, 2023
Due in one year or less$18.6 
Due after one year through five years243.4 
Due after five years through ten years378.7 
Due after ten years41.9 
Total debt securities$682.6