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Debt - Convertible Notes Narrative (Details)	3 Months Ended
	Mar. 31, 2021 USD ($) tradingDays $ / shares	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jul. 02, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 609,300,000		$ 641,400,000		$ 2,192,600,000	$ 2,193,200,000
Debt issuance costs	$ 15,100,000		14,500,000
Cap price (in dollars per share) | $ / shares	$ 19.76
Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,580,500,000		1,625,500,000		$ 1,592,100,000	$ 1,618,000,000.0
Additional Paid-in Capital [Member] | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Equity adjustment for adoption		$ (49,800,000)
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000		$ 291,400,000
Interest rate	3.50%		3.50%
Debt instrument, convertible, redemption price as a percent of principal amount	100.00%
Debt instrument, convertible, threshold percentage of stock price trigger	130.00%
Debt instrument, convertible, threshold trading days | tradingDays	20
Debt instrument, convertible, threshold consecutive trading days | tradingDays	30
Debt instrument, convertible, conversion ratio	0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 15.49
Debt instrument, convertible, number of equity instruments (in shares)	18,800,000
Equity component of convertible debt	$ 0		$ 46,800,000	$ 51,400,000
Debt issuance costs	$ 7,800,000
Debt instrument, interest rate, effective percentage	4.20%		8.40%
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Accounting Standards Update 2020-06
Debt Instrument [Line Items]
Equity adjustment for adoption		$ (45,400,000)
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Debt issuance costs			$ 1,600,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 84,200,000		$ 84,200,000
Interest rate	4.75%		4.75%
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, convertible, redemption price as a percent of principal amount	100.00%
Debt instrument, convertible, conversion ratio	0.0692042
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 14.45
Debt instrument, convertible, number of equity instruments (in shares)	5,800,000
Debt issuance costs	$ 600,000
Debt instrument, interest rate, effective percentage	5.40%

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