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Marketable Securities Held in Trusts (Tables)
 3 Months Ended
Mar. 31, 2024
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 March 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.
March 31, 2024
Due in one year or less$19.8 
Due after one year through five years234.2 
Due after five years through ten years376.3 
Due after ten years47.0 
Total debt securities$677.3 
Schedule of Unrealized Loss on Investments
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 March 31, 2024 and December 31, 2023:
March 31, 2024December 31, 2023
(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$20.4 $(0.2)$5.4 $(0.1)
Municipal bonds44.7 (0.4)42.3 (0.2)
U.S. government bonds57.2 — 26.4 (0.3)
$122.3 $(0.6)$74.1 $(0.6)
Securities that have been in a continuous loss position for more than 12 months:
Corporate debt securities$106.1 $(8.2)$121.5 $(8.3)
Municipal bonds99.1 (4.4)84.1 (3.9)
U.S. government bonds— — — — 
$205.2 $(12.6)$205.6 $(12.2)
Unrealized Gain (Loss) on Investments
The estimated fair value of the investments in the RCRA Trusts as of March 31, 2024 and December 31, 2023 are as follows:
March 31, 2024
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair
Value
Level 1
    Cash and cash equivalents $5.4 $— $— $5.4 
Level 2
    Corporate debt securities200.7 1.1 (8.4)193.4 
    Municipal bonds203.3 1.1 (4.8)199.6 
    U.S. government bonds284.1 0.2 — 284.3 
Total$693.5 $2.4 $(13.2)$682.7 
December 31, 2023
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