Debt Covenant, Undrawn Availability Requirement, Amount

No definition available.

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- Definition

Debt Covenant, Undrawn Availability Requirement, Percent Of Total Available Liquidity

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenant, Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

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- Definition

Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Minimum Borrowing Capacity

+ References

No definition available.

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- Definition

Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Percent Maximum Borrowing Capacity

+ References

No definition available.

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- Definition

Debt Instrument, Covenant, Minimum Liquidity, Number of Days Prior

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- Definition

Debt Instrument, Prepayment Increments, Minimum

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No definition available.

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- Definition

Line of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

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- Definition

Line of Credit Facility, Increase to Maximum Borrowing Capacity

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No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

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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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Average amount borrowed under the credit facility during the period.

+ References

No definition available.

+ Details

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- Definition

The effective interest rate during the reporting period.

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

No definition available.

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Debt - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Debt - Narrative (Details) - USD ($) $ in Millions	Mar. 29, 2026	Dec. 28, 2025	Mar. 30, 2025
Debt Instrument [Line Items]
Debt issuance costs	$ 10.9	$ 11.6
Credit Facility
Debt Instrument [Line Items]
Amount utilized to support the issuance of letters of credit	29.3
Average borrowings during period	$ 62.8
Average annual interest rate	5.40%
Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Minimum fixed charge coverage ratio allowed in event of default	1.00
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant	10.00%
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum	$ 60.0
Minimum required liquidity number of days prior to maturity of senior notes	90 days
Debt issuance costs	$ 2.8
Unamortized debt issuance expense	$ 1.9
Credit Facility \| Line of Credit \| Minimum \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable interest rate (percentage)	1.25%
Credit Facility \| Line of Credit \| Minimum \| Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)	0.25%
Credit Facility \| Line of Credit \| Maximum \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable interest rate (percentage)	1.75%
Credit Facility \| Line of Credit \| Maximum \| Base rate borrowings
Debt Instrument [Line Items]
Variable interest rate (percentage)	0.75%
ATI Inc. 5.875% Notes due 2027 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	5.875%
Debt instrument carrying amount	$ 350.0	350.0
ATI Inc. 4.875% Notes due 2029 \| Finance leases and other
Debt Instrument [Line Items]
Interest rate	4.875%
Debt instrument carrying amount	$ 325.0	325.0
Revolving credit facility \| Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	600.0
Line of credit facility, increase to maximum borrowing capacity	300.0
Debt covenant, undrawn availability requirement, amount	$ 120.0
Debt covenant, undrawn availability requirement, percent of total available liquidity	20.00%
Debt instrument carrying amount	$ 75.0	0.0	$ 0.0
Letter of credit sub-facility \| Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	200.0
Secured Debt \| Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	200.0
Line of credit facility, accordion feature, increase limit	$ 100.0
Variable interest rate (percentage)	2.00%
Minimum prepayment increments allowed	$ 25.0
Debt instrument carrying amount	200.0	200.0
Bridge Loan \| Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	60.0
Foreign credit facilities \| Foreign Credit Agreements
Debt Instrument [Line Items]
Maximum borrowing capacity	74.2
Foreign credit facilities \| Foreign Credit Agreements \| Line of Credit
Debt Instrument [Line Items]
Debt instrument carrying amount	$ 1.0	$ 0.0