Derivatives, Hedging Programs and Other Financial Instruments - Summary of Loss (Gain) Associated with Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	$ 2,348.1	$ 941.3	$ 1,215.2
Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded	91.5	52.2	49.1
Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded	(38.9)	(1.4)	(20.7)
(Gain) loss recognized in our Statements of Consolidated Income (Loss)		0.3
Cost of Products Sold | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	(3.8)	(3.3)
Cost of Products Sold | Not Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	(5.0)	0.2
Loss (gain) recognized in our Statements of Consolidated Income – Unrealized loss (gain)	1.2	(3.5)
Depreciation and Amortization | Designated as Hedging Instrument | Euro
Derivative Instruments Gain Loss [Line Items]
Foreign exchange contracts		(0.1)
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	2,348.1	941.3	1,215.2
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	(41.9)	17.7	19.7
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument | Electricity
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	(4.7)	0.5
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument | Aluminum
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	(35.2)	13.7	18.4
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument | Alloying Metals
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	0.1	1.4	1.1
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument | Alloying Metals (Reclassification Due To Forecasted Transactions No Longer Probable of Occurring)
Derivative Instruments Gain Loss [Line Items]
Alloying Metals (reclassification from AOCI due to forecasted transactions no longer probable of occurring)		0.8
Cash Flow Hedges | Cost of Products Sold | Designated as Hedging Instrument | Natural Gas
Derivative Instruments Gain Loss [Line Items]
(Gain) loss recognized in our Statements of Consolidated Income (Loss)	$ (2.1)	1.3	$ 0.2
Fair Value 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		52.2
Total of income and expense line items presented in our Statements of Consolidated Income (loss) in which the effects of cashflow hedges are recorded		1.4
Fair Value Hedges | Depreciation and Amortization | Designated as Hedging Instrument | Euro
Derivative Instruments Gain Loss [Line Items]
Hedged item		0.1
Excluded component amortized from AOCI		0.4
Fair Value Hedges | Other Expense, Net | Designated as Hedging Instrument | Euro
Derivative Instruments Gain Loss [Line Items]
Reclassification from AOCI related to early termination of firm commitment		0.6
Net settlement related to early termination of forward contracts		$ (0.7)