Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 29, 2024 USD ($)	Oct. 01, 2023 USD ($)	Sep. 29, 2024 USD ($) segment	Oct. 01, 2023 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jul. 02, 2023 USD ($)	Jan. 01, 2023 USD ($)
Revenue recognition [Line Items]
Number of business segments | segment			2
Unfavorable impact on pre-tax operating results	$ (114.9)	$ (99.0)	$ (311.1)	$ (287.1)
Equity adjustment for adoption	1,912.0	1,355.9	1,912.0	1,355.9	$ 1,467.9	$ 1,480.5	$ 1,309.2	$ 1,157.2
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,937.8	1,689.5	1,937.8	1,689.5	1,712.9	1,697.1	1,682.0	1,668.1
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ (72.8)	$ (215.8)	$ (72.8)	$ (215.8)	$ 78.4	$ (70.1)	$ (306.0)	$ (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%
Allegheny & Tsingshan Stainless
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%
Uniti
Revenue recognition [Line Items]
Equity method investment ownership percentage	50.00%		50.00%