Commitments and Contingencies (Details) $ in Millions			1 Months Ended	3 Months Ended
		Aug. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2026 USD ($) qtr laborUnion MW	Mar. 31, 2025 USD ($)	Feb. 13, 2026 USD ($)	Oct. 31, 2025	Nov. 30, 2024 potline
Hawesville [Abstract]
Partial withdrawal liability						$ 5.7
Insurance Contingencies [Abstract]
Number of potlines effected by transformer | potline								1
Number of potlines | potline								2
Reduction in production percentage							66.67%
Gain on insurance proceeds - net				$ 33.0	$ 0.0
Proceeds from insurance settlement				37.0
Insurance recovery expense	[1]			$ 4.0
Labor Commitments [Abstract]
Percentage of total work force in union				56.00%
Percentage of Grundartangi work force represented by the labor unions				87.00%
Number of labor unions Grundartangi subsidiary entered into a new labor agreement with | laborUnion				5
Percentage of domestic based work force represented by a union				35.00%
Percentage of foreign work force, represented by the union				63.00%
Contingent consideration, accrued interest and principal				$ 34.1
First 9 Quarters
Hawesville [Abstract]
Quarterly cost				$ 0.6
Number of quarters | qtr				9
Final Quarter
Hawesville [Abstract]
Quarterly cost				$ 0.5
Number of quarters | qtr				1
E ON Contingent Obligation | Long-Term Debt
Labor Commitments [Abstract]
Stated interest rate, percentage				11.00%
Netherlands
Labor Commitments [Abstract]
Percentage of total work force in union				100.00%
Kenergy | Century Marketer, LLC
Power Contingencies [Abstract]
Extension term				1 year
Required termination period				1 year
Sebree | Kenergy
Power Contingencies [Abstract]
Extension term				1 year
Required termination period				1 year
Santee Cooper
Power Contingencies [Abstract]
Power agreement, power supply, percentage				100.00%
Grundartangi - HS, Landsvirkjun and OR
Power Contingencies [Abstract]
Power currently available under the power purchase agreement, available (in megawatts) | MW				545
Grundartangi - Landsvirkjun
Power Contingencies [Abstract]
Power currently available under the power purchase agreement, available (in megawatts) | MW				25
Ravenswood Retiree Medical Benefits Changes
Ravenswood litigation [Abstract]
Litigation settlement amount		$ 23.0
Ravenswood litigation settlement installment period		10 years
Litigation payment to trust			$ 5.0
Gain (loss) related to litigation settlement			5.5
Loss contingency accrual			$ 12.5
Litigation settlement, annual installment				$ 2.0
Litigation settlement installment period				9 years
Other current liabilities				$ 1.9

[1] (1) Purchases from related party were $64.2 million and $88.3 million for the three months ended March 31, 2026 and 2025, respectively.