Selected Quarterly Financial Data (Unaudited) - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Unusual Or Infrequent Item [Line Items]
Gain (Loss) on Disposition of Oil and Gas and Timber Property			$ 91.7
Net gain (loss) on sale of businesses	$ 13.8	$ 0.0	(7.1)
Restructuring and other credits (charges)	10.5	(1,132.1)	(4.5)
Joint venture restructuring and impairment charges	0.0	(2.4)	(11.4)
Valuation allowance increase (decrease) due to change in judgment on the realizability of deferred tax assets			(45.1)
Impairment of goodwill	0.0	(287.0)	0.0
Severance Costs		(60.5)
Termination benefits		(17.4)
Other Asset Impairment Charges	0.0	(1,041.5)	0.0
Other Postretirement Benefits
Unusual Or Infrequent Item [Line Items]
Termination benefits	$ 0.0	$ (6.7)	0.0
Allegheny & Tsingshan Stainless
Unusual Or Infrequent Item [Line Items]
Joint venture restructuring and impairment charges			(11.4)
Equity method investment ownership percentage	50.00%
Forged Products
Unusual Or Infrequent Item [Line Items]
Net gain (loss) on sale of businesses			(13.3)
Cast Products
Unusual Or Infrequent Item [Line Items]
Net gain (loss) on sale of businesses			6.2
Other Asset Impairment Charges			$ (10.2)
Tsingshan Group \| Allegheny & Tsingshan Stainless
Unusual Or Infrequent Item [Line Items]
Equity method investment ownership percentage	50.00%

Other Asset Impairment Charges And Due From Affiliates, Allowance For Credit Loss

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Restructuring and other charges

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Amount of cost of providing special or contractual termination benefits payable from defined benefit plan.

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Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Amount of gain (loss) on sale or disposal of oil and gas property or timber property.

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Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

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