Debt - Convertible Notes Narrative (Details) $ / shares in Units, shares in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2022 USD ($) $ / shares	Jun. 30, 2022 USD ($) shares	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) tradingDays $ / shares	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($) $ / shares	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 938,400,000	$ 893,900,000	$ 680,200,000	$ 938,400,000	$ 680,200,000	$ 832,700,000	$ 606,100,000	$ 641,400,000
Debt issuance costs	18,000,000.0			18,000,000.0		20,800,000
Interest expense, net	(20,800,000)		$ (25,100,000)	(67,800,000)	(72,200,000)
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares		5.7
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security				16,600,000	$ 11,000,000.0
Conversion of convertible notes		$ 82,500,000		82,500,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	1,839,600,000			1,839,600,000		2,003,200,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	1,558,700,000			1,558,700,000		1,690,100,000
ATI Inc. 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 issuance costs	$ 5,200,000			$ 5,200,000		$ 6,500,000
Debt instrument, interest rate, effective percentage	4.20%		4.20%	4.20%	4.20%
Cap price (in dollars per share) | $ / shares	$ 19.76			$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 2,500,000		$ 2,500,000	$ 7,600,000	$ 7,600,000
Amortization of Debt Issuance Costs	400,000		500,000	1,300,000	1,300,000
Interest expense, net	2,900,000		$ 3,000,000.0	$ 8,900,000	$ 8,900,000
Debt instrument, convertible, number of equity instruments (in shares)				18,800,000
ATI Inc. 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	535,000,000			$ 535,000,000		379,000,000
ATI Inc. 4.75% Convertible Senior Notes due 2022
Debt Instrument [Line Items]
Debt, Gross	$ 0			$ 0		$ 84,200,000
Interest rate						4.75%
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security		$ 1,700,000
ATI Inc. 4.75% Convertible Senior Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Debt instrument, convertible, conversion ratio						0.0692042
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares						$ 14.45
Debt issuance costs						$ 300,000
Debt instrument, interest rate, effective percentage	5.40%		5.40%	5.40%	5.40%
Interest Expense, Debt, Excluding Amortization	$ 0		$ 1,000,000.0	$ 2,000,000.0	$ 3,000,000.0
Amortization of Debt Issuance Costs	0		200,000	300,000	400,000
Interest expense, net	$ 0		$ 1,200,000	$ 2,300,000	$ 3,400,000
ATI Inc. 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						$ 102,000,000