Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024		Dec. 31, 2023
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cashflow hedges are recorded		$ 2,930.6	$ 2,666.6	[1]	$ 2,728.2	[1]
Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	[2]		5.0		0.1
Not Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	[2]				0.1
Not Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	[2]		5.0
Cash Flow Hedges
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cashflow hedges are recorded		2,930.6	2,666.6	[2]	2,728.2	[2]
Cash Flow Hedges | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		$ (32.1)	$ (0.9)	[2]	$ 12.9	[2]
Derivative, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]		Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cashflow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cashflow hedges are recorded		Total of income and expense line items presented in our Statements of Consolidated Income in which the effects of cashflow hedges are recorded
Cash Flow Hedges | Designated as Hedging Instrument | Aluminum
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		$ (30.4)	$ (1.4)	[2]	$ 12.8	[2]
Cash Flow Hedges | Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		(2.3)	(1.1)	[2]
Cash Flow Hedges | Designated as Hedging Instrument | Foreign Currency
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		(0.2)
Cash Flow Hedges | Designated as Hedging Instrument | Natural Gas
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		$ 0.8	1.0	[2]	$ 0.1	[2]
Cash Flow Hedges | Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	[2]		$ 0.6

[1] Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 18 for further discussion.

[2] Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 18 for further discussion.