Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 143.6		$ 143.6		$ 205.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6.7)		(6.7)		(12.2)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	37.4		37.4		74.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2		0.2		0.6
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	86.3		86.3		121.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4.8)		(4.8)		(8.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	5.3		5.3		5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0		0.0		0.1
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	57.3		57.3		84.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1.9)		(1.9)		(3.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	25.7		25.7		42.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2		0.2		0.2
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	6.4		6.4		26.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0		0.0		$ 0.3
Debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-sale, Realized Loss	$ 1.2	$ 5.7	$ 10.4	$ 20.9

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