Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2022

Investments Debt And Equity Securities [Abstract]

Schedule of Marketable Securities

			Cost or
			amortized		Unrealized
	Market value		cost		loss, net
	(In millions)
December 31, 2021:
Current assets	$	2.6	$	2.6	$	—
Noncurrent assets	$	3.3	$	3.3	$	—
December 31, 2022:
Current assets	$	75.1	$	75.7	$	(.6)
Noncurrent assets	$	1.2	$	1.2	$	—

Schedule of Marketable Securities and Fair Value Measurements

	Fair Value Measurements
			Quoted		Significant
			Prices in		Other
			Active		Observable
			Markets		Inputs
	Total		(Level 1)		(Level 2)
	(In millions)
December 31, 2021:
Current assets - fixed income securities	$	2.6	$	—	$	2.6
Noncurrent assets:
Fixed income securities	$	1.3	$	—	$	1.3
Mutual funds		2.0		2.0		—
Total	$	3.3	$	2.0	$	1.3
December 31, 2022:
Current assets:
Fixed income securities	$	73.3	$	—	$	73.3
Mutual funds		1.8		1.8		—
Total	$	75.1	$	1.8	$	73.3
Noncurrent assets - fixed income securities	$	1.2	$	—	$	1.2