Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 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	$ 205.2		$ 205.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12.6)		(12.2)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	122.3		74.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.6		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	106.1		121.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8.2)		(8.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	20.4		5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2		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	99.1		84.1
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4.4)		(3.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	44.7		42.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.4		0.2
U.S. government 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	0.0		0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0		0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	57.2		26.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0		$ 0.3
Debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-sale, Realized Loss	$ 6.8	$ 13.4

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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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