Marketable Securities Held in Trusts - Continuous Loss Position (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 200.2		$ 200.2		$ 135.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(17.7)		(17.7)		(15.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	256.8		256.8		475.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3.8		3.8		12.9
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	114.9		114.9		72.8
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12.0)		(12.0)		(10.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	60.6		60.6		105.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.4		1.4		6.5
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	85.3		85.3		61.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5.7)		(5.7)		(5.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	84.0		84.0		104.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.2		1.2		2.9
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		0.8
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0		0.0		(0.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	112.2		112.2		264.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.2		1.2		$ 3.5
Debt securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss
Debt Securities, Available-for-sale, Realized Loss	$ 1.8	$ 24.9	$ 15.2	$ 25.9

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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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