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Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
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)

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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- 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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