Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 74.1	$ 475.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	12.2	15.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	205.6	135.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.6	12.9
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	5.4	105.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	8.3	10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	121.5	72.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1	6.5
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	42.3	104.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3.9	5.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	84.1	61.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2	2.9
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	26.4	264.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 0.3	$ 3.5

No definition available.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
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-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

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-Topic 320
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