Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 436.7	$ 74.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	8.4	12.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	136.9	205.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10.2	0.6
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	53.4	5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6.1	8.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	81.3	121.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.7	0.1
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	102.4	42.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2.3	3.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	55.6	84.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.7	0.2
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	280.9	26.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 7.8	$ 0.3

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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