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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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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 | $ 137.8 | $ 225.0
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (0.3) | (2.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 4.6 | 35.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.1) | (0.4)
Corporate Debt Securities |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 1.5 | 17.9
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0.0 | 20.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | (0.1)
Municipal Bonds |  |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 16.0 | 11.7
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (0.2) | (0.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 4.6 | 14.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (0.1) | (0.3)
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 | 120.3 | 195.4
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ (0.1) | $ (2.5)

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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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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 less than twelve months. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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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X
- 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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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
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Period Type: |

X
- Details
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