Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2023

Investments Debt And Equity Securities [Abstract]

Schedule of Marketable Securities

			Cost or
			amortized		Unrealized
	Market value		cost		loss, net
	(In millions)
December 31, 2022:
Current assets	$	75.1	$	75.7	$	(.6)
Noncurrent assets	$	1.2	$	1.2	$	—
September 30, 2023:
Current assets	$	83.2	$	83.4	$	(.2)
Noncurrent assets	$	3.2	$	3.6	$	(.4)