Equity Securities (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Summary of Investments in Equity Securities, by Accounting Classification

Investments in equity securities, by accounting classification, are summarized as follows:

	December 31,
(in millions)	2023		2022
Marketable equity securities	$	436.9	$	279.5
Non-marketable equity securities		224.1		395.8
Equity method investments		74.6		78.9
	$	735.6	$	754.2

Summary of Investments in Marketable Equity Securities

Investments in marketable equity securities are summarized as follows:

(in millions)	Cost		Unrealized losses			Estimated fair value
December 31, 2023
Common stocks	$	429.4	$	(4.9	)	$	424.5
Preferred stocks		15.7		(3.3	)		12.4
	$	445.1	$	(8.2	)	$	436.9
December 31, 2022
Common stocks	$	323.7	$	(55.6	)	$	268.1
Preferred stocks		15.3		(3.9	)		11.4
	$	339.0	$	(59.5	)	$	279.5

Summary of Changes in Carrying Amount of Non-Marketable Equity Securities

A summary of the changes in the carrying amount of non-marketable equity securities for the years ended December 31, 2023 and 2022, is as follows:

	Year ended December 31,
(in millions)	2023			2022
Carrying amount, beginning of period	$	395.8		$	441.3
Net (sales) additions		(4.0	)		91.8
Gross unrealized gains		1.5			16.1
Gross unrealized losses and impairments		(169.2	)		(153.4	)
Carrying amount, end of period	$	224.1		$	395.8