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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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For an unclassified balance sheet, amount of receivables due from an entity that is affiliated with the reporting entity by means of direct or indirect ownership.

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- Definition

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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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

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- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of cash inflow from a noncontrolling interest. Includes, but is not limited to, purchase of additional shares or other increase in noncontrolling interest ownership.

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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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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended		10 Months Ended	34 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 572.2		$ 490.8		$ 572.2
Income (loss) from equity investments	(1.0)			$ (2.4)	(6.6)	$ (8.7)
Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	$ 3.2				$ 3.2
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%				50.00%
Sales to noncontrolling interests							$ 12.0
Charges for termination benefits		$ 4.8
Impairment of Long-Lived Assets Held-for-use			14.2
Due From Affiliates, Allowance For Credit Loss			4.3
Allegheny & Tsingshan Stainless \| Employee Severance [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	$ 0.0	$ (2.4)		0.0	$ (2.4)	0.0
Uniti
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage	50.00%				50.00%
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 5.5				$ 5.5
Advanced Alloys & Solutions [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents	48.9				48.9
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	(1.6)			(2.6)	(10.1)	(9.9)
Due from Affiliates	14.5		32.5		14.5
Advanced Alloys & Solutions [Member] \| Uniti
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments	0.6			$ 0.2	1.1	$ 1.2
Forecast \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sales to noncontrolling interests								$ 17.5
Property, Plant and Equipment [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity investments			(7.1)
Prepaid expenses and other current assets \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	0.5		8.3		0.5
Other Noncurrent Assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from Affiliates	$ 14.0		$ 24.2		$ 14.0
Allegheny Technologies Inc \| Next Gen Alloys LLC
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	51.00%				51.00%
Allegheny Technologies Inc \| Advanced Alloys & Solutions [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage	60.00%				60.00%
China Baowu Steel Group Corporation Limited \| Advanced Alloys & Solutions [Member] \| China Baowu Steel Group Corporation Limited \| Shanghai STAL Precision Stainless Steel Co Ltd
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	40.00%				40.00%
Tsingshan Group \| Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%				50.00%
VSMPO \| VSMPO \| Uniti
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity	50.00%				50.00%