Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 103.6		$ 103.6		$ 136.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4.4)		(4.4)		(8.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	32.4		32.4		436.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2		0.2		10.2
Corporate debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	55.2		55.2		81.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3.1)		(3.1)		(6.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	8.7		8.7		53.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0		0.0		0.7
Municipal bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	48.4		48.4		55.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1.3)		(1.3)		(2.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	14.3		14.3		102.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2		0.2		1.7
U.S. government bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	9.4		9.4		280.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0		0.0		$ 7.8
Debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-sale, Realized Loss	0.8	$ 1.2	8.6	$ 10.4
Debt Securities, Available-for-sale, Realized Gain	$ 0.6	$ 6.4	$ 1.3	$ 17.2

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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.

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