Debt Schedule of Debt (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (19,900,000)	$ (20,800,000)
Debt	1,824,100,000	1,842,900,000
Short-term debt and current portion of long-term debt	116,700,000	131,300,000
Long-term debt	1,707,400,000	1,711,600,000
Allegheny Technologies 5.875% Notes due 2027
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	Allegheny Technologies	Allegheny Technologies
Interest rate	5.875%	5.875%
Debt Instrument, Maturity Date	Dec. 01, 2027	Dec. 01, 2027
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000	$ 291,400,000
Debt Issuance Costs, Net	$ (6,100,000)	$ (6,500,000)
Debt Instrument, Issuer	Allegheny Technologies	Allegheny Technologies
Interest rate	3.50%	3.50%
Debt Instrument, Maturity Date	Jun. 15, 2025	Jun. 15, 2025
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 84,200,000	$ 84,200,000
Debt Instrument, Issuer	Allegheny Technologies	Allegheny Technologies
Interest rate	4.75%	4.75%
Debt Instrument, Maturity Date	Jul. 01, 2022	Jul. 01, 2022
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 \| Convertible Debt
Debt Instrument [Line Items]
Debt Issuance Costs, Net	$ (100,000)	$ (300,000)
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
2024 Term Loan
Debt Instrument [Line Items]
Debt, Gross	$ 200,000,000	$ 200,000,000.0
Debt Instrument, Issuer	Allegheny Technologies	Allegheny Technologies
Interest rate	4.21%
Debt Instrument, Maturity Date	Sep. 30, 2024	Sep. 30, 2024
Domestic Bank Group $500 million asset-based credit facility
Debt Instrument [Line Items]
Short-term Debt	$ 0	$ 0
Foreign credit facilities
Debt Instrument [Line Items]
Short-term Debt	12,700,000	27,400,000
Finance leases and other
Debt Instrument [Line Items]
Debt, Gross	80,700,000	85,700,000
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross	$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer	Allegheny Technologies	Allegheny Technologies
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	Allegheny Technologies	Allegheny Technologies
Interest rate	4.875%	4.875%
Debt Instrument, Maturity Date	Oct. 01, 2029	Oct. 01, 2029

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