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Debt - Schedule of Debt (Details) - USD ($)	12 Months Ended
Debt - Schedule of Debt (Details) - USD ($)	Dec. 29, 2024	Dec. 31, 2023	Mar. 31, 2024	Aug. 11, 2023	Sep. 09, 2022	Sep. 08, 2022	Jan. 02, 2022
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ 14,200,000	$ 19,600,000
Total short-term and long-term debt	1,895,300,000	2,179,600,000
Short-term debt and current portion of long-term debt	180,400,000	31,900,000
Total long-term debt	1,714,900,000	2,147,700,000
ATI $350 million 5.875% Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
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 interest rate stated percentage							4.75%
Allegheny Ludlum 6.95% Debentures due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	6.95%	6.95%
Debt instrument carrying amount	$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Issuer	Allegheny Ludlum	Allegheny Ludlum
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
ATI 2031Notes
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date	Oct. 01, 2031	Oct. 01, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 325,000,000.0	$ 325,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029
2027 Term Loan
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage			4.21%
Debt instrument carrying amount	$ 200,000,000	$ 200,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
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
ATI 2030 Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	7.25%	7.25%
Debt instrument carrying amount	$ 425,000,000.0	$ 425,000,000.0
Debt Issuance Costs, Net				$ 6,200,000
Debt instrument, term	7 years
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date	Aug. 15, 2030	Aug. 15, 2030
Finance leases and other [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 109,500,000	$ 102,800,000
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount	0	$ 5,000,000.0
Convertible Debt [Member] \| ATI Convertible Senior Notes, 3.5%, Due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage		3.50%
Debt instrument carrying amount	$ 0	$ 291,400,000
Debt Issuance Costs, Net		$ 2,900,000
Debt Instrument, Issuer		ATI Inc.
Debt Instrument, Maturity Date		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