Commitments and Contingencies (Details) R$ in Millions						3 Months Ended		12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2025 BRL (R$)	Jun. 30, 2025 USD ($)	Jun. 30, 2025 BRL (R$)	Mar. 22, 2022 USD ($)	Dec. 31, 2025 USD ($) claim	Dec. 31, 2024 USD ($)	Dec. 31, 2025 USD ($) case claim	Dec. 31, 2025 BRL (R$) claim	Jan. 17, 2025 company	Jul. 19, 2024 EUR (€)	Nov. 30, 2022 USD ($)	Feb. 25, 2022 EUR (€)	Oct. 14, 2021 EUR (€)	Dec. 30, 2020 USD ($)
Commitments and Contingencies
Proposed civil penalty												$ 800,000
Accrual for Environmental Loss Contingencies						$ 27,400,000		$ 27,400,000
Penalties per day | €													€ 5,000	€ 100,000
EnvironmentalLossContingencyStatementOfFinancialPositionExtensibleEnumerationNotDisclosedFlag								Company’s results of operations
Penalties						6,000,000		$ 6,000,000	R$ 32.0
Current Portion of Environmental Liabilities						$ 12,736,000	$ 11,815,000	$ 12,736,000
Number Of Potentially Related Parties | company										30
Other
Commitments and Contingencies
Number of pending claims | claim						17,000		17,000	17,000
Loss provision								$ 0
Loss Contingency, Claims Dismissed, Number | case								28,500
Unfavorable Regulatory Action
Commitments and Contingencies
Loss provision						$ 5,000,000.0	$ 27,200,000
Violations of the Federal Emergency Planning and Community Right-to-Know Act | Pending Litigation
Commitments and Contingencies
Liabilities for asserted and unasserted claims						600,000		$ 600,000
Loss Contingency, Damages Sought, Value					$ 3,000,000.0
Limit and Reduce the Accumulation of Customer-Owned Slag
Commitments and Contingencies
Loss Contingency, Damages Sought, Value			$ 12,000,000	R$ 66.0
Limit and Reduce the Accumulation of Customer-Owned Slag | CSN
Commitments and Contingencies
Loss Contingency, Damages Sought, Value			48,000,000	R$ 264.0
Brazil
Commitments and Contingencies
Loss provision								0
Loss Contingencies, Unpaid Service Taxes															$ 1,900,000
Loss Contingencies, Overall Liability						$ 6,800,000		$ 6,800,000
Kentucky Department of Environmental Protection
Commitments and Contingencies
Proposed civil penalty	$ 1,200,000
Current Portion of Environmental Liabilities			$ 200,000
Amsterdam Public Prosecutor’s Office | Alleged Violation of Environmental Permit in Ijmuiden, Netherland
Commitments and Contingencies
Loss Contingency, Fine Issued By Court | €											€ 25,000
Volta Redonda vs. CSN and Enviri | Limit and Reduce the Accumulation of Customer-Owned Slag | Pending Litigation
Commitments and Contingencies
Loss Contingency, Damages Sought, Value	$ 78,000,000	R$ 431.0