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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 172.6 |  | $ 172.6 |  | $ 205.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (11.3) |  | (11.3) |  | (12.2)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 386.0 |  | 386.0 |  | 74.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 4.4 |  | 4.4 |  | 0.6
Corporate debt securities |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 99.4 |  | 99.4 |  | 121.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (8.0) |  | (8.0) |  | (8.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 34.3 |  | 34.3 |  | 5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.3 |  | 0.3 |  | 0.1
Municipal bonds |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 73.2 |  | 73.2 |  | 84.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (3.3) |  | (3.3) |  | (3.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 86.5 |  | 86.5 |  | 42.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1.1 |  | 1.1 |  | 0.2
U.S. government bonds |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |  |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 265.2 |  | 265.2 |  | 26.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 3.0 |  | 3.0 |  | $ 0.3
Debt securities |  |  |  |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |  |  |  |
Debt Securities, Available-for-sale, Realized Loss | $ 7.9 | $ 1.8 | $ 14.7 | $ 15.2 |

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