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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale securities, less than twelve months, fair value | $ 302.6 | $ 137.8
Available-for-sale securities, less than twelve months, gross unrealized losses | (7.7) | (0.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 7.0 | 4.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.1) | (0.1)
Corporate debt securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale securities, less than twelve months, fair value | 44.9 | 1.5
Available-for-sale securities, less than twelve months, gross unrealized losses | (1.0) | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Municipal bonds |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale securities, less than twelve months, fair value | 35.9 | 16.0
Available-for-sale securities, less than twelve months, gross unrealized losses | (0.5) | (0.2)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 7.0 | 4.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.1) | (0.1)
U.S. government bonds |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss |  |
Available-for-sale securities, less than twelve months, fair value | 221.8 | 120.3
Available-for-sale securities, less than twelve months, gross unrealized losses | (6.2) | (0.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 0.0 | $ 0.0

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References No definition available. + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References No definition available. + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer. + References No definition available. + Details
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