Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Carrying Value Reported Amount Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 432.9	$ 584.0
Derivative financial instruments, Assets	4.2	18.0
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	2,205.9	1,765.2
Estimate Of Fair Value Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	432.9	584.0
Derivative financial instruments, Assets	4.2	18.0
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	2,359.3	1,964.5
Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	432.9	584.0
Derivative financial instruments, Assets	0.0	0.0
Derivative Financial Instruments, liabilities	0.0	0.0
Debt	2,044.8	1,665.7
Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Derivative financial instruments, Assets	4.2	18.0
Derivative Financial Instruments, liabilities	9.3	4.6
Debt	314.5	298.8
Convertible Debt | ATI Inc. 3.5% Convertible Senior Notes due 2025 | Estimate Of Fair Value Fair Value Disclosure | Fair Value Inputs Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt	$ 786.0	$ 590.0