Debt - Convertible Notes Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) $ / shares	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) tradingDays $ / shares	Jun. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)	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	$ 606,100,000	$ 1,842,300,000	$ 606,100,000	$ 1,842,300,000	$ 609,300,000		$ 641,400,000		$ 2,192,600,000	$ 2,193,200,000
Debt issuance costs	14,200,000		14,200,000				14,500,000
Interest expense, net	(23,700,000)	(21,700,000)	(47,100,000)	(43,600,000)
Debt extinguishment charge (See Note 7)	0	(21,500,000)	0	(21,500,000)
Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Equity adjustment for adoption	1,587,500,000	$ 1,617,800,000	1,587,500,000	$ 1,617,800,000	$ 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				$ 291,400,000
Interest rate	3.50%		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		$ 15.49
Debt instrument, convertible, number of equity instruments (in shares)			18,800,000
Equity component of convertible debt	$ 0		$ 0				$ 46,800,000	$ 51,400,000
Debt issuance costs	$ 7,400,000		$ 7,400,000				$ 8,200,000
Debt instrument, interest rate, effective percentage	4.20%		4.20%				8.40%
Cap price (in dollars per share) | $ / shares	$ 19.76		$ 19.76
Interest expense, net	$ 2,900,000		$ 5,900,000
Amortization of Debt Issuance Costs	400,000		800,000
Interest Expense, Debt, Excluding Amortization	2,500,000		5,100,000
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				$ 84,200,000
Interest rate	4.75%		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		$ 14.45
Debt instrument, convertible, number of equity instruments (in shares)			5,800,000
Debt issuance costs	$ 500,000		$ 500,000				$ 700,000
Debt instrument, interest rate, effective percentage	5.40%	5.40%	5.40%	5.40%
Interest expense, net	$ 1,100,000	$ 3,600,000	$ 2,200,000	$ 7,400,000
Amortization of Debt Issuance Costs	100,000	400,000	200,000	800,000
Interest Expense, Debt, Excluding Amortization	$ 1,000,000.0	$ 3,200,000	$ 2,000,000.0	$ 6,600,000
Debt instrument, repurchased face amount							$ 203,200,000