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Amount of short-term and long-term debt and lease obligation.

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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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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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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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Debt - Schedule of Debt (Details) - USD ($) $ in Millions	Dec. 28, 2025	Dec. 15, 2025	Jun. 13, 2025	Dec. 29, 2024	Aug. 31, 2023
Debt Instrument [Line Items]
Finance lease liabilities	$ 111.0			$ 109.5
Debt issuance costs	(11.6)			(14.2)
Total short-term and long-term debt	1,749.4			1,895.3
Short-term debt and current portion of long-term debt	31.1			180.4
Long-term debt	$ 1,718.3			1,714.9
ATI Inc. 7.25% Senior Notes due 2030 \| Notes
Debt Instrument [Line Items]
Interest rate	7.25%				7.25%
Debt instrument carrying amount	$ 425.0			425.0	$ 425.0
Debt issuance costs					$ (6.2)
ATI Inc. 5.875% Senior Notes due 2027 \| Notes
Debt Instrument [Line Items]
Interest rate	5.875%		5.875%
Debt instrument carrying amount	$ 350.0			350.0
ATI Inc. 5.125% Senior Notes due 2031 \| Notes
Debt Instrument [Line Items]
Interest rate	5.125%
Debt instrument carrying amount	$ 350.0			350.0
ATI Inc. 4.875% Senior Notes due 2029 \| Notes
Debt Instrument [Line Items]
Interest rate	4.875%		4.875%
Debt instrument carrying amount	$ 325.0			325.0
Allegheny Ludlum 6.95% Debentures due 2025
Debt Instrument [Line Items]
Interest rate	6.95%	6.95%
Debt instrument carrying amount	$ 0.0	$ 150.0		150.0
Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Debt issuance costs			$ (2.8)
Credit Facility \| Line of Credit \| Secured Debt
Debt Instrument [Line Items]
Debt instrument carrying amount	200.0			200.0
Credit Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument carrying amount	0.0			0.0
Foreign Credit Agreements \| Line of Credit \| Foreign Line of Credit
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 0.0			$ 0.0