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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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

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Identification of the contractual issuer of the debt instrument (for example, a subsidiary of the reporting entity).

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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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No definition available.

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Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of long-term debt and lease obligation, classified as noncurrent.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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Debt Schedule of Debt (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Debt Schedule of Debt (Details) - USD ($)	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020	Jul. 02, 2020	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (22,800,000)	$ (22,800,000)	$ (14,500,000)
Total Debt	2,290,500,000	2,290,500,000	1,567,800,000
Short-term debt and current portion of long-term debt	606,900,000	606,900,000	17,800,000
Long-term debt	1,683,600,000	1,683,600,000	1,550,000,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 500,000,000	$ 500,000,000	$ 500,000,000.0
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	7.875%	7.875%	7.875%		7.875%	5.875%
Debt Instrument, Maturity Date		Aug. 15, 2023	Aug. 15, 2023
Allegheny Technologies 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	5.875%	5.875%	5.875%
Debt Instrument, Maturity Date		Dec. 01, 2027	Dec. 01, 2027
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	(7,000,000)	(7,000,000)	(8,200,000)
Equity component of convertible debt	$ 0	$ 0	$ (46,800,000)	$ (51,400,000)
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	3.50%	3.50%	3.50%
Debt Instrument, Maturity Date		Jun. 15, 2025	Jun. 15, 2025
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 84,200,000	$ 84,200,000	$ 84,200,000
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	4.75%	4.75%	4.75%
Debt Instrument, Maturity Date		Jul. 01, 2022	Jul. 01, 2022
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 \| Convertible Debt
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (400,000)	$ (400,000)	$ (700,000)
Allegheny Ludlum 6.95% Debentures due 2025 (b)
Debt Instrument [Line Items]
Debt, Gross	$ 150,000,000.0	$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Issuer		Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%	6.95%
Debt Instrument, Maturity Date		Dec. 15, 2025	Dec. 15, 2025
2024 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000.0	$ 200,000,000.0	$ 200,000,000.0
Debt Instrument, Issuer		Allegheny Technologies	Allegheny Technologies
Interest rate	4.21%	4.21%
Debt Instrument, Maturity Date		Sep. 30, 2024	Sep. 30, 2024
Domestic Bank Group $500 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 0	$ 0
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	9,200,000	9,200,000	5,500,000
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	53,500,000	53,500,000	48,000,000.0
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000	$ 350,000,000	0
Debt Instrument, Issuer	Allegheny Technologies
Interest rate	5.125%	5.125%
Debt Instrument, Maturity Date		Oct. 01, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325,000,000.0	$ 325,000,000.0	$ 0
Debt Instrument, Issuer	Allegheny Technologies
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date		Oct. 01, 2029