Debt Schedule of Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 30, 2025	Dec. 29, 2024
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (13.5)	$ (14.2)
Debt	1,893.2	1,895.3
Short-term debt and current portion of long-term debt	179.9	180.4
Long-term debt	1,713.3	1,714.9
ATI Inc. 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350.0	$ 350.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.875%	5.875%
Allegheny Ludlum 6.95% Debentures due 2025 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 150.0	$ 150.0
Debt Instrument, Issuer	Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	$ 0.0	$ 0.0
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	106.7	109.5
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350.0	$ 350.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.125%	5.125%
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325.0	$ 325.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.875%	4.875%
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0.0	$ 0.0
2027 Term Loan
Debt Instrument [Line Items]
Debt, Gross	200.0	200.0
ATI 2030 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 425.0	$ 425.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	7.25%	7.25%

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

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