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

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Amount of consolidated retained earnings that represent undistributed earnings of 50% or less owned by persons accounted for by the equity method

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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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Amount of gain (loss) from deconsolidation of subsidiary and derecognition of group of assets constituting transfer of business or nonprofit activity, excluding conveyance of oil and gas mineral rights and transfer of good or service in contract with customer.

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

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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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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		10 Months Ended	12 Months Ended			34 Months Ended
Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest				$ 0.0	$ 0.0	$ 14.4
Gain on joint venture deconsolidation				0.0	0.0	15.9
Cash and cash equivalents		$ 490.8		645.9	490.8		$ 645.9
Income (loss) from equity method investments				(7.0)	(10.7)	(1.0)
Undistributed earnings of investees accounted for under equity method				12.7			12.7
Non-cash joint venture impairment charge	$ 2.4	11.4		2.4	11.4	0.0
Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Cash and cash equivalents				38.3			38.3
Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest						2.7
Cash and cash equivalents				$ 2.7			$ 2.7
Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage				50.00%			50.00%
Impairment of long-lived assets held-for-use					14.2
Income (loss) from equity method investments				$ (10.6)	(19.3)	(3.9)
Revenue from related parties				18.4	14.6	4.1
Related party accounts receivable		0.1		0.0	0.1		$ 0.0
Due from affiliates, allowance for credit loss		4.3			4.3
Non-cash joint venture impairment charge					11.4
Business Exit Costs				$ 4.8
Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment ownership percentage				50.00%			50.00%
Revenue from related parties				$ 36.7	31.3	49.4
Related party accounts receivable		0.2		1.4	0.2		$ 1.4
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates				5.5			5.5
Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				(8.2)	(12.2)
Due from affiliates		36.8		14.0	36.8		14.0
Advanced Alloys & Solutions [Member] \| Uniti [Member]
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments				1.2	1.5	$ 2.9
Prepaid expenses and other current assets [Member] \| Advanced Alloys & Solutions [Member] \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Due from affiliates		8.3		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		$ 28.5		$ 13.5	28.5		$ 13.5
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage				60.00%			60.00%
Allegheny Technologies Inc \| Next Gen Alloys LLC [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage				51.00%			51.00%
Allegheny Technologies Inc \| China Baowu Steel Group Corporation Limited [Member] \| Shanghai STAL Precision Stainless Steel Co Ltd [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture ownership percentage by unaffiliated entity				40.00%			40.00%
Property, Plant and Equipment \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Income (loss) from equity method investments					$ (7.1)
Tsingshan Group \| Allegheny & Tsingshan Stainless
Schedule of Equity Method Investments [Line Items]
Sale to noncontrolling interest			$ 12.0				$ 17.5
Equity method investment ownership percentage				50.00%			50.00%