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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | $ 56.2 | $ 363.8
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | [1] | (0.6) | (5.2)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 220.9 | 183.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | [1] | (7.1) | (4.1)
Corporate Debt Securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 43.9 | 44.3
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | [1] | (0.6) | (0.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 103.4 | 100.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | [1] | (3.7) | (1.9)
Municipal Bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 12.3 | 64.5
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | [1] | 0.0 | (0.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 117.5 | 83.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | [1] | (3.4) | (2.2)
U.S. Government Bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 0.0 | 255.0
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | [1] | 0.0 | (4.4)
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 | [1] | $ 0.0 | $ 0.0
[1] | Represents the aggregate of the gross unrealized losses that have been in a continuous unrealized loss position as of December 31, 2018 and December 31, 2017.

X
- 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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- 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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