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

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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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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	Jun. 28, 2026	Jun. 08, 2026	Jun. 03, 2026	Dec. 28, 2025	Jun. 29, 2025
Debt Instrument [Line Items]
Finance leases and other	$ 106.4			$ 111.0
Debt issuance costs	(17.1)			(11.6)
Debt	2,192.0			1,749.4
Short-term debt and current portion of long-term debt	383.6			31.1
Long-term debt	$ 1,808.4			1,718.3
ATI Inc. 7.25% Senior Notes due 2030 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	7.25%
Debt instrument carrying amount	$ 425.0			425.0
ATI Inc. 5.875% Senior Notes due 2033 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate			5.875%
Debt instrument carrying amount	$ 450.0			0.0
Debt issuance costs			$ (6.9)
ATI Inc. 5.875% Senior Notes due 2027 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	5.875%	5.875%
Debt instrument carrying amount	$ 350.0	$ 350.0		350.0
ATI Inc. 5.125% Senior Notes due 2031 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	5.125%
Debt instrument carrying amount	$ 350.0			350.0
ATI Inc. 4.875% Senior Notes due 2029 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	4.875%
Debt instrument carrying amount	$ 325.0			325.0
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	$ 0.0
Foreign Credit Agreements \| Line of Credit \| Foreign credit facilities
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 2.7			$ 0.0