Financing Arrangements - Narrative (Details) shares in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2025 USD ($) shares	Mar. 31, 2025 USD ($) Days shares	Mar. 31, 2024 USD ($) shares	Dec. 31, 2024 USD ($)	May 06, 2024 USD ($)	Nov. 02, 2022 USD ($)	Dec. 20, 2021 shares
Debt Instrument [Line Items]
Interest paid		$ 300,000	$ 300,000
Common Stock, Shares outstanding \| shares							50.0
Average Repurchase price		14.23	20.87
Money market investment carrying value		42,600,000	146,400,000
Cash held in other investments		$ 126,900,000	$ 121,300,000
Stock Repurchase Program, Amount						$ 75,000,000
Common Shares
Debt Instrument [Line Items]
Stock Repurchased, Shares \| shares		0.4	0.2
Stock Repurchased, Amount		$ 5,600,000	$ 400,000
Stock Repurchase Program, Remaining Amount		97,200,000
Stock Repurchase Program, Amount					$ 100,000,000
Subsequent Event
Debt Instrument [Line Items]
Average Repurchase price	$ 12.47
Subsequent Event \| Common Shares
Debt Instrument [Line Items]
Stock Repurchased, Shares \| shares	0.1
Stock Repurchased, Amount	$ 1,200,000
Stock Repurchase Program, Remaining Amount	$ 96,000,000
Convertible Notes
Debt Instrument [Line Items]
Final cash payment of interest made to noteholders		100,000	200,000
Credit Agreement \| Credit Agreement
Debt Instrument [Line Items]
Outstanding borrowings			0
Line of credit facility, tentative future commitment increase			251,700,000
Convertible Senior Notes due 2025
Debt Instrument [Line Items]
Outstanding borrowings		0
Principal amount		$ 5,500,000		$ 5,500,000
Transaction costs, debt gross			$ 1,500,000
Terms of conversion			The Convertible Senior Notes due 2025 are convertible at the option of holders in certain circumstances and during certain periods into the Company’s common shares, cash, or a combination thereof, at the Company’s election.
Debt instrument, threshold trading days \| Days		20
Debt Instrument, threshold consecutive trading days \| Days		30
Debt Instrument, threshold Percentage of stock price conversion		130.00%
Fair value of convertible notes		$ 9,700,000
Convertible Senior Notes due 2025 \| Convertible Notes
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 46,000,000

Metallus Inc. 10-Q