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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | $ 436.7 | $ 74.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 8.4 | 12.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 136.9 | 205.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 10.2 | 0.6
Corporate Debt Securities |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 53.4 | 5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 6.1 | 8.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 81.3 | 121.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.7 | 0.1
Municipal Bonds |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 102.4 | 42.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 2.3 | 3.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 55.6 | 84.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1.7 | 0.2
U.S. Government Bonds |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | 280.9 | 26.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | 0.0 | 0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 7.8 | $ 0.3

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