Debt Schedule of Debt (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 29, 2024	Dec. 31, 2023	Mar. 31, 2024
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (14,800,000)	$ (19,600,000)
Debt	1,883,400,000	2,179,600,000
Short-term debt and current portion of long-term debt	27,900,000	31,900,000
Long-term debt	1,855,500,000	2,147,700,000
ATI Inc. 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.875%	5.875%
Debt Instrument, Maturity Date	Dec. 01, 2027	Dec. 01, 2027
ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 0	$ 291,400,000
Debt Issuance Costs, Net		$ (2,900,000)
Debt Instrument, Issuer		ATI Inc.
Interest rate		3.50%
Debt Instrument, Maturity Date		Jun. 15, 2025
Allegheny Ludlum 6.95% Debentures due 2025 (a)
Debt Instrument [Line Items]
Debt, Gross	$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Issuer	Allegheny Ludlum	Allegheny Ludlum
Interest rate	6.95%	6.95%
Debt Instrument, Maturity Date	Dec. 15, 2025	Dec. 15, 2025
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 5,000,000.0
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	98,200,000	102,800,000
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	5.125%	5.125%
Debt Instrument, Maturity Date	Oct. 01, 2031	Oct. 01, 2031
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 325,000,000.0	$ 325,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 0
2027 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000.0	$ 200,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate			4.21%
Debt Instrument, Maturity Date	Sep. 09, 2027	Sep. 09, 2027
ATI 2030 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 425,000,000.0	$ 425,000,000.0
Debt Instrument, Issuer	ATI Inc.	ATI Inc.
Interest rate	7.25%	7.25%
Debt Instrument, Maturity Date	Aug. 15, 2030	Aug. 15, 2030

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