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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) segment	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment	2
Impact of new accounting principle on prior period operating income	$ 112.6	$ 77.0
Unfavorable impact on pre-tax operating results	(76.5)	(40.1)
Equity adjustment for adoption	1,214.3	795.7	$ 1,157.2	$ 832.7
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,675.1	1,602.5	1,668.1	1,596.7
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 247.0	$ 103.6	$ 176.9	$ 72.7
ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Revenue recognition [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.50%		3.50%
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%