total of both long-term debt, short-term debt and current portion of long-term debt

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Amount of debt and lease obligation, classified as current.

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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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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 ($)	9 Months Ended	12 Months Ended
Debt Schedule of Debt (Details) - USD ($)	Sep. 30, 2022	Dec. 31, 2021	Sep. 09, 2022	Sep. 08, 2022
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (18,000,000.0)	$ (20,800,000)
Debt	1,729,300,000	1,842,900,000
Short-term debt and current portion of long-term debt	28,900,000	131,300,000
Long-term debt	1,700,400,000	1,711,600,000
ATI Inc. 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.875%	5.875%
Debt Instrument, Maturity Date	Dec. 01, 2027	Dec. 01, 2027
ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	$ (5,200,000)	$ (6,500,000)
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	3.50%	3.50%
Debt Instrument, Maturity Date	Jun. 15, 2025	Jun. 15, 2025
ATI Inc. 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 0	$ 84,200,000
Debt Instrument, Issuer		ATI Inc.
Interest rate		4.75%
Debt Instrument, Maturity Date		Jul. 01, 2022
ATI Inc. 4.75% Convertible Senior Notes due 2022 \| Convertible Debt
Debt Instrument [Line Items]
Debt Issuance Costs, Net		$ (300,000)
Allegheny Ludlum 6.95% Debentures due 2025 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Issuer	Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	$ 8,500,000	$ 27,400,000
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	72,400,000	85,700,000
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.125%	5.125%
Debt Instrument, Maturity Date	Oct. 01, 2031	Oct. 01, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325,000,000.0	$ 325,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 0
2027 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000.0	$ 200,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.21%
Debt Instrument, Maturity Date	Sep. 09, 2027	Sep. 09, 2027
Revolving credit facility \| Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Debt Issuance Costs, Net			$ (2,400,000)	$ (1,700,000)