Derivatives, Hedging Programs and Other Financial Instruments - Summary of (Gain) Loss Associated with Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	$ 690.5	$ 718.4	$ 1,333.4	$ 1,449.5
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 (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded
Loss recognized in our Statements of Consolidated Income - Unrealized mark-to-market loss			$ 2.2
Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	$ 2.2	$ 0.2	2.2	$ 0.1
Not Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated Income - Unrealized mark-to-market loss		0.2		0.1
Not Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated Income - Unrealized mark-to-market loss	2.0		2.0
Reclassification from AOCI due to forecasted transactions probable of not occurring	0.2		0.2
Cash Flow Hedges
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated Income (Loss) in which the effects of cash flow hedges are recorded	690.5	718.4	1,333.4	1,449.5
Cash Flow Hedges | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	(2.0)	5.5	0.1	5.7
Cash Flow Hedges | Designated as Hedging Instrument | Aluminum
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	(2.7)	5.2	(0.7)	$ 5.7
Cash Flow Hedges | Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	(0.5)		(0.5)
Cash Flow Hedges | Designated as Hedging Instrument | Natural Gas
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	0.4	$ 0.3	0.7
Cash Flow Hedges | Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income	$ 0.8		$ 0.6