Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Apr. 02, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jan. 01, 2023 USD ($)
Revenue recognition [Line Items]
Number of business segments | segment	2
Impact of new accounting principle on prior period operating income	$ 115.2	$ 112.6
Unfavorable impact on pre-tax operating results	(85.3)	(90.9)
Equity adjustment for adoption	1,373.2	1,214.3	$ 1,480.5	$ 1,157.2
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,703.1	1,675.1	1,697.1	1,668.1
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ (4.0)	$ (396.4)	$ (70.1)	$ (480.9)
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%