Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Carrying Value Reported Amount Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 394.4	$ 743.9
Derivative financial instruments, Assets	0.5	0.9
Derivative Financial Instruments, liabilities	10.7	14.2
Debt	2,192.0	2,199.2
Estimate Of Fair Value Fair Value Disclosure
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	394.4	743.9
Derivative financial instruments, Assets	0.5	0.9
Derivative Financial Instruments, liabilities	10.7	14.2
Debt	2,836.8	2,746.7
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	394.4	743.9
Derivative financial instruments, Assets	0.0	0.0
Derivative Financial Instruments, liabilities	0.0	0.0
Debt	2,536.2	2,438.9
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	0.5	0.9
Derivative Financial Instruments, liabilities	10.7	14.2
Debt	300.6	307.8
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	$ 967.0	$ 864.0