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Debt - Convertible Notes Narrative (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) tradingDays $ / shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Jul. 02, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Debt extinguishment charge		$ 21,500,000	$ 0	$ 21,500,000	$ 0
Payment for Debt Extinguishment or Debt Prepayment Cost		19,100,000		19,100,000	0
Debt issuance costs				8,100,000	0
Purchase of convertible note capped call				(19,000,000.0)	$ 0
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 issuance costs	$ 16,100,000	16,100,000		16,100,000			$ 12,300,000
Equity component of convertible note		$ 48,700,000		$ 48,700,000
Cap price (in dollars per share) | $ / shares	$ 19.76	$ 19.76		$ 19.76
[AdditionalPaidInCapitalMember]
Debt Instrument [Line Items]
Equity component of convertible note		$ 48,700,000		$ 48,700,000
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt
Debt Instrument [Line Items]
Interest rate	3.50%	3.50%		3.50%
Debt instrument, face amount, additional amount offered to underwriters	$ 40,000,000.0	$ 40,000,000.0		$ 40,000,000.0
Debt instrument, convertible, conversion ratio	0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 15.49	$ 15.49		$ 15.49
Debt instrument, interest rate, effective percentage	8.40%	8.40%		8.40%
Debt, Gross	$ 285,000,000.0	$ 285,000,000.0		$ 285,000,000.0			0
Equity component of convertible debt	50,300,000	50,300,000		50,300,000			0
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt | Long-term Debt [Member]
Debt Instrument [Line Items]
Debt issuance costs	7,300,000	7,300,000		7,300,000
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt | Additional Paid in Capital
Debt Instrument [Line Items]
Debt issuance costs	1,600,000	1,600,000		1,600,000
Allegheny Technologies, Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt | Subsequent Event
Debt Instrument [Line Items]
Debt, Gross						$ 6,400,000
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 84,200,000	$ 84,200,000		$ 84,200,000			$ 287,500,000
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 | Convertible Debt
Debt Instrument [Line Items]
Interest rate	4.75%	4.75%		4.75%
Debt instrument, repurchased face amount	$ 203,200,000	$ 203,200,000		$ 203,200,000
Write off of Deferred Debt Issuance Cost		$ 2,400,000

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