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Marketable Securities Held in Trusts (Tables) | 12 Months Ended
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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 securities | 204.6 |  |  | 1.9 |  |  | (8.4) |  |  | 198.1 |
Municipal bonds | 206.9 |  |  | 1.9 |  |  | (4.1) |  |  | 204.7 |
U.S. government bonds | 268.6 |  |  | 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 securities | 203.8 |  |  | 0.1 |  |  | (17.1) |  |  | 186.8 |
Municipal bonds | 197.0 |  |  | 0.4 |  |  | (8.0) |  |  | 189.4 |
U.S. government bonds | 269.6 |  |  | — |  |  | (3.6) |  |  | 266.0 |
Other holdings | 0.2 |  |  | — |  |  | — |  |  | 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, 2023 |  | December 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 bonds | 42.3 |  |  | (0.2) |  |  | 104.7 |  |  | (2.9) |
U.S. government bonds | 26.4 |  |  | (0.3) |  |  | 264.9 |  |  | (3.5) |
Total | $ | 74.1 |  |  | $ | (0.6) |  |  | $ | 475.2 |  |  | $ | (12.9) |
| December 31, 2023 |  | December 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 bonds | 84.1 |  |  | (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 years | 243.4 |
Due after five years through ten years | 378.7 |
Due after ten years | 41.9 |
Total debt securities | $ | 682.6 |

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