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

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 ($)	12 Months Ended
Debt - Schedule of Debt (Details) - USD ($)	Dec. 31, 2022	Dec. 31, 2021	Sep. 09, 2022	Sep. 08, 2022	Dec. 31, 2020	Jul. 02, 2020
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ 17,200,000	$ 20,800,000
Total short-term and long-term debt	1,748,000,000	1,842,900,000
Short-term debt and current portion of long-term debt	41,700,000	131,300,000
Total long-term debt	$ 1,706,300,000	$ 1,711,600,000
ATI $350 million 5.875% Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument issuer	ATI Inc.	ATI Inc.
Debt instrument interest rate stated percentage	5.875%	5.875%
Debt instrument carrying amount	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Maturity Date	Dec. 01, 2027	Dec. 01, 2027
ATI $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument issuer		ATI Inc.
Debt instrument interest rate stated percentage		4.75%
Debt instrument carrying amount	$ 0	$ 84,200,000
Debt Instrument, Maturity Date		Jul. 01, 2022
Allegheny Ludlum 6.95% Debentures due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument issuer	Allegheny Ludlum	Allegheny Ludlum
Debt instrument interest rate stated percentage	6.95%	6.95%
Debt instrument carrying amount	$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
ATI 2031Notes
Debt Instrument [Line Items]
Debt instrument issuer	ATI Inc.	ATI Inc.
Debt instrument interest rate stated percentage	5.125%	5.125%
Debt instrument carrying amount	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Maturity Date	Oct. 01, 2031	Oct. 01, 2031
Debt instrument, term	10 years
ATI 2029 Notes
Debt Instrument [Line Items]
Debt instrument issuer	ATI Inc.	ATI Inc.
Debt instrument interest rate stated percentage	4.875%	4.875%
Debt instrument carrying amount	$ 325,000,000.0	$ 325,000,000.0
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029
Debt instrument, term	8 years
2027 Term Loan
Debt Instrument [Line Items]
Debt instrument issuer	ATI Inc.	ATI Inc.
Debt instrument interest rate stated percentage	4.21%
Debt instrument carrying amount	$ 200,000,000	$ 200,000,000.0
Debt Instrument, Maturity Date	Sep. 30, 2027	Sep. 30, 2027
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Short-Term Debt	$ 0	$ 0
Finance leases and other [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	79,400,000	85,700,000
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 19,400,000	$ 27,400,000
Convertible Debt [Member] \| ATI $287.5 million Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage		4.75%
Debt Issuance Costs, Net		$ 300,000
Convertible Debt [Member] \| ATI Convertible Senior Notes, 3.5%, Due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument issuer	ATI Inc.	ATI Inc.
Debt instrument interest rate stated percentage	3.50%	3.50%
Debt instrument carrying amount	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	$ 4,800,000	$ 6,500,000
Equity component of convertible debt					$ (46,800,000)	$ (51,400,000)
Debt Instrument, Maturity Date	Jun. 15, 2025	Jun. 15, 2025
Revolving Credit Facility [Member] \| Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Debt Issuance Costs, Net			$ 2,400,000	$ 1,700,000