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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)
Revenue recognition [Line Items]
Number of business segments \| segment			2
Impact of new accounting principle on prior period operating income	$ 113.2	$ 29.9	$ 178.6	$ 50.4
Unfavorable impact on pre-tax operating results	(67.4)	(77.2)	(76.6)	(39.9)
Equity adjustment for adoption	938.4	680.2	938.4	680.2	$ 893.9	$ 832.7	$ 606.1		$ 641.4
Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption	1,662.2	1,592.1	1,662.2	1,592.1	1,656.6	1,596.7	1,587.5		1,625.5
[RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption	$ 100.0	$ 102.5	$ 100.0	$ 102.5	$ 38.9	$ 72.7	$ 53.8		$ 106.5
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%			3.50%
Accounting Standards Update 2020-06 \| Additional Paid-in Capital [Member]
Revenue recognition [Line Items]
Equity adjustment for adoption								$ (49.8)
Accounting Standards Update 2020-06 \| [RetainedEarningsMember]
Revenue recognition [Line Items]
Equity adjustment for adoption								4.4
Accounting Standards Update 2020-06 \| ATI Inc. 3.5% Convertible Senior Notes due 2025 \| Convertible Debt
Revenue recognition [Line Items]
Equity adjustment for adoption								$ (45.4)
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%