Marketable Securities Held in Trusts (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Debt Securities, Available-for-sale

The estimated fair value of the investments in the RCRA Trusts as of December 31, 2023 and December 31, 2022 are as follows:

	December 31, 2023
(in millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Level 1
Cash and cash equivalents	$	1.0	$	—	$	—	$	1.0
Level 2
Corporate debt securities	204.6		1.9		(8.4)		198.1
Municipal bonds	206.9		1.9		(4.1)		204.7
U.S. government bonds	268.6		11.5		(0.3)		279.8
Total	$	681.1	$	15.3	$	(12.8)	$	683.6
	December 31, 2022
(in millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
Level 1
Cash and cash equivalents	$	7.7	$	—	$	—	$	7.7
Level 2
Corporate debt securities	203.8		0.1		(17.1)		186.8
Municipal bonds	197.0		0.4		(8.0)		189.4
U.S. government bonds	269.6		—		(3.6)		266.0
Other holdings	0.2		—		—		0.2
Total	$	678.3	$	0.5	$	(28.7)	$	650.1

Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value

The following tables show gross unrealized losses and fair values of the RCRA Trusts’ available-for-sale securities that have been in a continuous unrealized loss position for which an allowance for credit losses has not been recorded as of December 31, 2023 and December 31, 2022.

	December 31, 2023				December 31, 2022
Securities that have been in a continuous loss position for less than 12 months (in millions):	Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses
Corporate debt securities	$	5.4	$	(0.1)	$	105.6	$	(6.5)
Municipal bonds	42.3		(0.2)		104.7		(2.9)
U.S. government bonds	26.4		(0.3)		264.9		(3.5)
Total	$	74.1	$	(0.6)	$	475.2	$	(12.9)
	December 31, 2023				December 31, 2022
Securities that have been in a continuous loss position for more than 12 months (in millions):	Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses
Corporate debt securities	$	121.5	$	(8.3)	$	72.8	$	(10.6)
Municipal bonds	84.1		(3.9)		61.9		(5.1)
U.S. government bonds	—		—		0.8		(0.1)
Total	$	205.6	$	(12.2)	$	135.5	$	(15.8)

Schedule of maturity dates for debt securities

The following table summarizes the balance by contractual maturity of the available-for-sale debt securities invested by the RCRA Trusts as of December 31, 2023. Actual maturities may differ from contractual maturities because the issuers of the securities may have the right to prepay obligations before the underlying contracts mature.

(in millions)	December 31, 2023
Due in one year or less	$	18.6
Due after one year through five years	243.4
Due after five years through ten years	378.7
Due after ten years	41.9
Total debt securities	$	682.6