Derivatives, Hedging Programs and Other Financial Instruments - Summary of Loss (Gain) Associated with Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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 cashflow hedges are recorded	$ 2,754.9	$ 3,180.2	$ 2,348.1
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 cashflow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded	Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded
Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)	$ 0.1	$ 0.9	$ (3.8)
Not Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)	0.1	(0.5)	(5.0)
Loss (gain) recognized in our Statements of Consolidated Income - Unrealized loss		1.4	1.2
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 cashflow hedges are recorded	2,754.9	3,180.2	2,348.1
Cash Flow Hedges | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)	12.9	(10.5)	(41.9)
Cash Flow Hedges | Designated as Hedging Instrument | Aluminum
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)	12.8	7.4	(35.2)
Cash Flow Hedges | Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)			0.1
Cash Flow Hedges | Designated as Hedging Instrument | Natural Gas
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)	$ 0.1	(6.6)	(2.1)
Cash Flow Hedges | Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
Loss (gain) recognized in our Statements of Consolidated Income (Loss)		$ (11.3)	$ (4.7)