Retirement Benefits - Schedule of Participation in Multiemployer Plans (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023
Multiemployer Plan Disclosure [Line Items]
Company Contributions	$ 6.7	$ 5.9	$ 5.2
Steelworkers Western Independent Shops Pension Plan
Multiemployer Plan Disclosure [Line Items]
Company Contributions	1.3	1.1	0.7
Boilermakers-Blacksmiths National Pension Trust
Multiemployer Plan Disclosure [Line Items]
Company Contributions	3.1	2.7	2.6
IAM National Pension Fund
Multiemployer Plan Disclosure [Line Items]
Company Contributions	$ 2.3	$ 2.1	$ 1.9

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

ATI INC 10-K