Debt - Convertible Notes Narrative (Details)	3 Months Ended
	Mar. 31, 2022 USD ($) tradingDays $ / shares	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 795,700,000	$ 609,300,000	$ 832,700,000	$ 641,400,000
Debt issuance costs	19,900,000		20,800,000
Interest expense, net	(23,600,000)	$ (23,400,000)
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,148,300,000		2,003,200,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	1,854,900,000		1,690,100,000
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 issuance costs	$ 6,100,000		$ 6,500,000
Debt instrument, interest rate, effective percentage	4.20%	4.20%
Cap price (in dollars per share) | $ / shares	$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 2,500,000	$ 2,500,000
Amortization of Debt Issuance Costs	400,000	400,000
Interest expense, net	$ 2,900,000	$ 2,900,000
Debt instrument, convertible, number of equity instruments (in shares)	18,800,000
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	$ 550,000,000
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 issuance costs	$ 100,000		$ 300,000
Debt instrument, interest rate, effective percentage	5.40%	5.40%
Interest Expense, Debt, Excluding Amortization	$ 1,000,000.0	$ 1,000,000.0
Amortization of Debt Issuance Costs	100,000	100,000
Interest expense, net	$ 1,100,000	$ 1,100,000
Debt instrument, convertible, number of equity instruments (in shares)	5,800,000
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	$ 160,000,000