Entity Number Of Employees Covered By USW Collective Bargaining Agreements Expiring February 29 2020

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Adjustment to additional paid in capital resulting from the recognition of convertible debt instruments as two separate components - a debt component and an equity component. This bifurcation may result in a basis difference associated with the liability component that represents a temporary difference for purposes of applying accounting for income taxes. The initial recognition of deferred taxes for the tax effect of that temporary difference is as an adjustment to additional paid in capital.

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Contractual interest rate for funds borrowed, under the debt agreement.

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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 income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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

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- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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The net result for the period of deducting operating expenses from operating revenues.

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Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Jan. 01, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) employee	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment			2
Impact of new accounting principle on prior period operating income	$ (9.6)	$ 94.5	$ (214.6)	$ 267.6
Unfavorable impact on pre-tax operating results	46.2	(119.0)	339.1	(220.4)
Entity Number Of Employees Covered By USW Collective Bargaining Agreements Expiring February 29 2020 \| employee							1,500
Equity component of convertible note	1.1		49.8
Equity adjustment for adoption	1,849.8	2,279.9	1,849.8	2,279.9		$ 1,842.3	$ 2,193.2	$ 2,138.6	$ 1,991.6
[AdditionalPaidInCapitalMember]
Revenue recognition [Line Items]
Equity component of convertible note	1.1		49.8
Equity adjustment for adoption	1,622.2	1,612.1	1,622.2	1,612.1		1,617.8	1,618.0	1,604.6	1,615.4
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 1,227.4	$ 1,622.8	$ 1,227.4	$ 1,622.8		$ 1,277.6	$ 1,679.3	$ 1,511.8	$ 1,422.0
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Revenue recognition [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Forecast \| Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt \| [RetainedEarningsMember] \| Cumulative Effect, Period of Adoption, Adjustment
Revenue recognition [Line Items]
Equity adjustment for adoption					$ (4.6)
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Allegheny Technologies Inc \| Shanghai STAL Precision Stainless Steel Co Ltd \| Advanced Alloys & Solutions [Member]
Revenue recognition [Line Items]
Joint venture ownership percentage	60.00%		60.00%