Business Segments - Reconciliation of Net Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	$ 649.2	$ 633.9
Selling, general and administrative expenses	(25.8)	(12.5)
Gain on the sale of Hawesville	287.9	0.0
Other operating expense - net	(6.9)	(2.0)
Interest income	3.1	1.8
Gain on insurance proceeds - net	33.0	0.0
Other loss - net	(5.5)	(3.4)
Income tax expense	(1.8)	(1.6)
Net income	327.0	22.4
Nonrelated Party
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	343.3	255.2
Interest expense	(9.9)	(10.0)
Net loss on forward and derivative contracts - nonaffiliates	(65.3)	(5.4)
Related Party
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	305.9	378.7
Interest expense	(0.6)	(1.8)
Reportable Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	649.2	633.9
Segment Cost of goods sold	(537.1)	(574.8)
IRA Credit	25.0	19.9
Lower of cost or NRV inventory adjustment	3.3	3.1
Property and equipment expense	(21.6)	(24.8)
Selling, general and administrative expenses	(25.8)	(12.5)
Gain on the sale of Hawesville	287.9	0.0
Other operating expense - net	(6.9)	(2.0)
Interest income	3.1	1.8
Gain on insurance proceeds - net	33.0	0.0
Other loss - net	(5.5)	(3.4)
Income tax expense	(1.8)	(1.6)
Net income	327.0	22.4
Reportable Segment | Nonrelated Party
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	(9.9)	(10.0)
Net loss on forward and derivative contracts - nonaffiliates	(65.3)	(5.4)
Reportable Segment | Related Party
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	$ (0.6)	$ (1.8)