Debt - Credit Agreements (Details)		12 Months Ended
	Jun. 13, 2025 USD ($)	Dec. 28, 2025 USD ($)	Dec. 29, 2024 USD ($)	Jun. 30, 2024 USD ($)
Debt Instrument [Line Items]
Debt issuance costs, net		$ 11,600,000	$ 14,200,000
Credit Facility
Debt Instrument [Line Items]
Line of credit facility amount outstanding		29,300,000
Average outstanding amount		$ 2,600,000
Interest rate during period		6.50%
Credit Facility | Line of Credit
Debt Instrument [Line Items]
Fixed charge coverage ratio	1.00
Remaining borrowing capacity, percent of minimum borrowing capacity	10.00%
Minimum remaining borrowing capacity	$ 60,000,000.0
Demonstrate minimum liquidity period	90 days
Debt issuance costs, net	$ 2,800,000
Unamortized debt issuance expense	$ 1,900,000
Credit Facility | Line of Credit | Minimum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread	1.25%
Credit Facility | Line of Credit | Minimum | Base Rate
Debt Instrument [Line Items]
Variable rate spread	0.25%
Credit Facility | Line of Credit | Maximum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread	1.75%
Credit Facility | Line of Credit | Maximum | Base Rate
Debt Instrument [Line Items]
Variable rate spread	0.75%
ABL Term Loan
Debt Instrument [Line Items]
Interest rate				4.21%
ABL Term Loan | Interest rate swap
Debt Instrument [Line Items]
Derivative notional amount				$ 50,000,000
ATI $350 million 5.875% Senior Notes due 2027 | Notes
Debt Instrument [Line Items]
Interest rate	5.875%	5.875%
Debt instrument carrying amount		$ 350,000,000.0	350,000,000.0
ATI Inc. 4.875% Senior Notes due 2029 | Notes
Debt Instrument [Line Items]
Interest rate	4.875%	4.875%
Debt instrument carrying amount		$ 325,000,000.0	325,000,000.0
Revolving Credit Facility | Credit Facility
Debt Instrument [Line Items]
Fixed charge coverage ratio		1.00
Revolving Credit Facility | Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 600,000,000
Line of credit facility, increase to maximum borrowing capacity	300,000,000
Debt covenant, undrawn availability requirement, amount	$ 120,000,000
Debt covenant, undrawn availability requirement, percent of total available liquidity	20.00%
Debt instrument carrying amount		$ 0	0
Letter of Credit | Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	$ 200,000,000
Bridge Loan | Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	60,000,000
Secured Debt | Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity	200,000,000
Line of credit facility, accordion feature, increase limit	$ 100,000,000
Variable rate spread	2.00%
Debt instrument, prepayment increments, minimum	$ 25,000,000
Debt instrument carrying amount		200,000,000.0	200,000,000.0
Foreign Line of Credit | Foreign Credit Agreements
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity		73,000,000
Foreign Line of Credit | Foreign Credit Agreements | Line of Credit
Debt Instrument [Line Items]
Debt instrument carrying amount		$ 0	$ 0