v3.25.4
Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2025
Feb. 13, 2026
Jun. 30, 2025
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2025    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Trading Symbol MTUS    
Title of 12(b) Security Common shares    
Security Exchange Name NYSE    
Entity Registrant Name METALLUS INC.    
Entity Central Index Key 0001598428    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Common Stock, Shares Outstanding   41,730,421  
Entity Public Float     $ 616,943,911
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Shell Company false    
Document Financial Statement Error Correction [Flag] false    
ICFR Auditor Attestation Flag true    
Document Annual Report true    
Document Transition Report false    
Entity Incorporation, State or Country Code OH    
Entity Address, Address Line One 1835 Dueber Avenue SW    
Entity Address, City or Town Canton    
Entity Address, State or Province OH    
City Area Code 330    
Local Phone Number 471.7000    
Entity Address, Postal Zip Code 44706    
Entity Tax Identification Number 46-4024951    
Entity File Number 1-36313    
Documents Incorporated by Reference

DOCUMENTS INCORPORATED BY REFERENCE

Document

Parts Into Which Incorporated

Proxy Statement for the 2026 Annual Meeting of Shareholders

Part III
   
Auditor Name Ernst & Young LLP    
Auditor Firm ID 42    
Auditor Location Cleveland, Ohio    
Auditor Opinion [Text Block]

Opinion on the Financial Statements

We have audited the accompanying consolidated balance sheets of Metallus Inc. (the Company) as of December 31, 2025 and 2024, the related consolidated statements of operations, comprehensive income (loss), shareholders' equity and cash flows for each of the three years in the period ended December 31, 2025, and the related notes and the financial statement schedule listed in the Index at Item 15a (collectively referred to as the “consolidated financial statements”). In our opinion, the consolidated financial statements present fairly, in all material respects, the financial position of the Company at December 31, 2025 and 2024, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2025, in conformity with U.S. generally accepted accounting principles.

We also have audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (PCAOB), the Company's internal control over financial reporting as of December 31, 2025, based on criteria established in Internal Control-Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (2013 framework), and our report dated February 20, 2026, expressed an unqualified opinion thereon.