Debt - Convertible Notes Narrative (Details)	3 Months Ended			9 Months Ended
	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($) tradingDays $ / shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) tradingDays $ / shares	Sep. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jul. 02, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Feb. 15, 2016	Dec. 31, 2013
Debt Instrument [Line Items]
Payment for early debt extinguishment				$ 0	$ 19,100,000
Debt issuance costs				9,300,000	9,100,000
Equity adjustment for adoption		$ 680,200,000	$ 1,849,800,000	680,200,000	1,849,800,000	$ 606,100,000		$ 641,400,000		$ 1,842,300,000	$ 2,193,200,000
Debt issuance costs		22,800,000		22,800,000				14,500,000
Interest expense, net		(25,100,000)	(25,100,000)	(72,200,000)	(68,700,000)
Debt extinguishment charge		0	0	0	21,500,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		2,558,700,000		2,558,700,000				1,847,700,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		2,296,000,000		2,296,000,000				1,594,200,000
Forecast
Debt Instrument [Line Items]
Payment for early debt extinguishment	$ 64,500,000
Debt extinguishment charge	65,500,000
Additional Paid-in Capital [Member]
Debt Instrument [Line Items]
Equity adjustment for adoption		1,592,100,000	$ 1,622,200,000	1,592,100,000	$ 1,622,200,000	$ 1,587,500,000		1,625,500,000		$ 1,617,800,000	$ 1,618,000,000
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		$ 7,000,000		$ 7,000,000				8,200,000
Debt instrument, interest rate, effective percentage		4.20%	8.40%	4.20%	8.40%
Cap price (in dollars per share) | $ / shares		$ 19.76		$ 19.76
Interest Expense, Debt, Excluding Amortization		$ 2,500,000	$ 2,400,000	$ 7,600,000	$ 2,400,000
Amortization of Debt Issuance Costs		500,000	400,000	1,300,000	400,000
Interest expense, net		3,000,000.0	$ 2,800,000	8,900,000	$ 2,800,000
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		402,000,000		402,000,000
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		$ 400,000		$ 400,000				$ 700,000
Debt instrument, interest rate, effective percentage		5.40%	5.40%	5.40%	5.40%
Interest Expense, Debt, Excluding Amortization		$ 1,000,000.0	$ 1,000,000.0	$ 3,000,000.0	$ 7,600,000
Amortization of Debt Issuance Costs		200,000	100,000	400,000	900,000
Interest expense, net		1,200,000	$ 1,100,000	3,400,000	$ 8,500,000
Debt instrument, repurchased face amount								203,200,000
Allegheny Technologies 4.75% Convertible Senior Notes due 2022 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt		115,000,000		115,000,000
Allegheny Technologies 5.875% Notes due 2023 (a)
Debt Instrument [Line Items]
Debt, Gross		$ 500,000,000		$ 500,000,000				$ 500,000,000.0
Interest rate		7.875%		7.875%				7.875%				7.875%	5.875%
Debt instrument, notice period required for redemption | tradingDays		30
Allegheny Technologies 5.875% Notes due 2023 (a) | Forecast
Debt Instrument [Line Items]
Write off of deferred debt issue costs	$ 1,000,000
ATI 2029 and 2031 Notes
Debt Instrument [Line Items]
Debt issuance costs		$ 4,700,000
Debt instrument, redemption price, percentage		101.00%
Proceeds from Debt, Net of Issuance Costs		$ 665,700,000
ATI 2029 and 2031 Notes | Minimum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption | tradingDays		15
ATI 2029 and 2031 Notes | Maximum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption | tradingDays		60
ATI 2029 Notes
Debt Instrument [Line Items]
Debt, Gross		$ 325,000,000.0		$ 325,000,000.0				$ 0
Interest rate		4.875%		4.875%
Debt instrument, amortization period		8 years
ATI 2031 Notes
Debt Instrument [Line Items]
Debt, Gross		$ 350,000,000		$ 350,000,000				$ 0
Interest rate		5.125%		5.125%
Debt instrument, amortization period		10 years