Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Aerospace & Defense: Aerospace and Defense Market [Member] Aerospace and Defense Market [Member] Trading Arrangement: Trading Arrangement [Axis] Maximum Maximum [Member] Amortization to net income of net prior service (credits) costs Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Statistical Measurement [Domain] Statistical Measurement [Domain] Title of 12(b) Security Title of 12(b) Security Subsequent Event Subsequent Event [Member] Total Carrying Amount Reported Value Measurement [Member] Document Fiscal Year Focus Document Fiscal Year Focus Other related costs Other Restructuring Costs Additional authorized amount Share Repurchase Program, Additional Authorized Amount Share Repurchase Program, Additional Authorized Amount Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Adjustment to Compensation, Amount Adjustment to Compensation Amount Amounts in beginning balance reclassified to revenue Contract with Customer, Liability, Noncurrent, Reclassified to Revenue Contract with Customer, Liability, Noncurrent, Reclassified to Revenue Customer [Axis] Customer [Axis] Segment Reporting [Abstract] Segment Reporting [Abstract] Finance leases and other Finance Lease Liability And Other Finance Lease Liability And Other Common stock, par value $0.10: authorized-500,000,000 shares; issued-142,871,688 shares at June 28, 2026 and 142,871,688 shares at December 28, 2025; outstanding-136,168,724 shares at June 28, 2026 and 135,934,852 shares at December 28, 2025 Common Stock, Value, Issued Retirement Benefits Retirement Benefits [Text Block] Segment EBITDA Segment Earnings Before Income Taxes, Depreciation, Amortization (EBITDA) Segment Earnings Before Income Taxes, Depreciation, Amortization (EBITDA) Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Derivative Instrument [Axis] Derivative Instrument [Axis] Post- retirement benefit plans Accumulated Defined Benefit Plans Adjustment Attributable to Noncontrolling Interest [Member] Proceeds from sales of businesses, net of transaction costs Proceeds from Divestiture of Businesses Balance as of beginning of year Balance as of period end Accounts Receivable, Allowance for Credit Loss Retirement benefits Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits Losses on the Sale of Customer Accounts Receivable Losses on the Sale of Customer Accounts Receivable [Member] Losses on the Sale of Customer Accounts Receivable Amendment Flag Amendment Flag Jet Engines - Commercial Jet Engines- Commercial [Member] Jet Engines- Commercial Current Liabilities: Liabilities, Current [Abstract] Expense to increase the reserve Accounts Receivable, Credit Loss Expense (Reversal) Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Minimum required liquidity number of days prior to maturity of senior notes Debt Instrument, Covenant, Minimum Liquidity, Number of Days Prior Debt Instrument, Covenant, Minimum Liquidity, Number of Days Prior Repayment of finance lease obligations Finance Lease, Principal Payments Disposal Group Classification [Axis] Disposal Group Classification [Axis] Derivatives Derivatives AOCI, Derivative Qualifying as Hedge, Excluded Component, Parent [Member] Derivative financial instruments: Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract] Receivable purchase and financing agreement, maximum amount Receivables Purchase And Financing Agreement, Maximum Amount Receivables Purchase And Financing Agreement, Maximum Amount Rent and royalty income Rent, Royalty, And Other Income Rent, Royalty, And Other Income Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Minimum Borrowing Capacity Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Minimum Borrowing Capacity Minimum utilization level Receivables Purchase And Financing Agreement, Minimum Utilization, Percentage Receivables Purchase And Financing Agreement, Minimum Utilization, Percentage Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Medical Medical Market [Member] Medical Market [Member] Property, Plant, and Equipment [Abstract] Property, Plant, and Equipment [Abstract] Entity Address, State or Province Entity Address, State or Province PEO Total Compensation Amount PEO Total Compensation Amount Total Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Common stock, par value (USD per share) Common Stock, Par or Stated Value Per Share Reclassification out of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Member] Capital expenditures Segment Reporting, Long-Lived Asset, Expenditure for Addition Denominator for basic net income per common share – weighted average shares Weighted Average Number of Shares Outstanding, Basic Currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract] Derivatives AOCI, Derivative Qualifying as Hedge, Excluded Component, Noncontrolling Interest [Member] Short-term contract assets Balance as of beginning of year Balance as of period end Contract with Customer, Asset, after Allowance for Credit Loss, Current Retirement Benefits [Abstract] Retirement Benefits [Abstract] Inventory valuation reserves Inventory Valuation Reserves Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Fair Value By Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Reasonably possible amount by which current matters may exceed reserves Loss Contingency, Range of Possible Loss, Portion Not Accrued Number of reportable segments Number of Reportable Segments Total Current Liabilities Liabilities, Current Non-cash restructuring charges, net Non-Cash Restructuring Charges, Net Non-Cash Restructuring Charges, Net Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Pre-tax cash flow hedge loss to be reclassified within twelve months Cash Flow Hedge Gain (Loss) to be Reclassified within 12 Months Entity Address, City or Town Entity Address, City or Town Sales to external customers External Customers [Member] External Customers [Member] Income before income taxes Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Specialty Energy Specialty Energy [Member] Specialty Energy Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of Accounting Basis of Accounting, Policy [Policy Text Block] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Airframes - Commercial Airframes- Commercial [Member] Airframes- Commercial ATI Inc. 5.875% Senior Notes due 2033 ATI 2033 Notes ATI 2033 Senior Notes [Member] ATI 2033 Senior Notes ATI Inc. 5.125% Senior Notes due 2031 ATI 2031 Senior Notes [Member] ATI 2031 Senior Notes Gross accounts receivable Accounts Receivable, before Allowance for Credit Loss Amounts in beginning balance reclassified to revenue Contract with Customer, Liability, Current, Reclassified to Revenue Contract with Customer, Liability, Current, Reclassified to Revenue Significant Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Receivable [Domain] Receivable [Domain] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Other comprehensive income (loss), net of tax Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Other Restructuring Other Restructuring [Member] United States UNITED STATES Confirmed orders with current performance obligations Revenue, Remaining Performance Obligation, Current Revenue, Remaining Performance Obligation, Current Subsequent Event Type [Axis] Subsequent Event Type [Axis] Total Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Treasury stock: 6,702,964 shares at June 28, 2026 and 6,936,836 shares at December 28, 2025 Treasury Stock, Value Pending Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Accrued capital expenditures Capital Expenditures Incurred but Not yet Paid Schedule of Sales and Profit by Segment Segment Reporting [Table Text Block] Non- controlling Interests Attributable to noncontrolling interests: Noncontrolling Interest [Member] Net gain from disposal of property, plant and equipment Gain (Loss) on Disposition of Property Plant Equipment Related Party [Axis] Related and Nonrelated Parties [Axis] Less: Income taxes on postretirement benefit plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Proceeds from debt, net of issuance costs Proceeds from Debt, Net of Issuance Costs Property Plant and Equipment Property, Plant, and Equipment [Text Block] Document Period End Date Document Period End Date Cash used in investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Aggregate principal amount Debt Instrument, Face Amount Security Exchange Name Security Exchange Name Debt instrument, term Debt Instrument, Term Schedule of Other Financial Information By Segment Segment Reporting, Reconciliation of Other Item by Segment to Consolidated [Table Text Block] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Accumulated other comprehensive loss, net of tax Accumulated Other Comprehensive Income (Loss), Net of Tax Entity Shell Company Entity Shell Company Consolidation Items [Domain] Consolidation Items [Domain] Schedule of Property, Plant and Equipment, Net Property, Plant, and Equipment [Table Text Block] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Birmingham, UK and Dusseldorf, Germany Birmingham, UK and Dusseldorf, Germany [Member] Birmingham, UK and Dusseldorf, Germany Per Share Information Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Earnings Per Share [Abstract] Earnings Per Share [Abstract] Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Document Fiscal Period Focus Document Fiscal Period Focus Intersegment sales Internal Customers [Member] Internal Customers [Member] Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Total Liabilities Liabilities Trading Symbol Trading Symbol Geographical [Domain] Geographical [Domain] Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Financing Receivable, Allowance for Credit Loss [Line Items] Financing Receivable, Allowance for Credit Loss [Line Items] Reclassification to short-term Contract with Customer, Liability, Noncurrent, Reclassification Contract with Customer, Liability, Noncurrent, Reclassification Company Selected Measure Name Company Selected Measure Name Electronics Electronics Market [Member] Electronics Market [Member] Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Nickel Nickel [Member] Nickel [Member] Effect of dilutive securities: Earnings Per Share, Diluted [Abstract] Proceeds from sales of businesses, net of transaction costs Proceeds from Divestiture of Businesses, Net of Cash Divested Joint venture ownership percentage Subsidiary, Ownership Percentage, Parent Post- retirement benefit plans Postretirement benefit plans Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Debt instrument carrying amount Long-Term Debt, Gross Credit Facility [Domain] Credit Facility [Domain] HPMC High Performance Materials & Components [Member] High Performance Materials & Components [Member] Operating Activities: Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Compensation Amount Outstanding Recovery Compensation Amount Natural gas contracts Natural Gas Contracts [Member] Derivative instrument whose primary underlying risk is tied to commodity prices for natural gas Amortization of contract costs Capitalized Contract Cost, Amortization Individual: Individual [Axis] Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions Weighted Average Number of Shares Outstanding, Diluted Loss on sales of businesses Gain (Loss) on Sale of Assets and Asset Impairment Charges Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Entity Emerging Growth Company Entity Emerging Growth Company Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Disposal Group Name [Domain] Disposal Group Name [Domain] Derivatives Derivatives, Policy [Policy Text Block] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Deferred taxes Deferred Income Taxes and Tax Credits Derivative Financial Instruments and Hedging Derivative Instruments and Hedging Activities Disclosure [Text Block] Restructuring charges (credits) Restructuring charges (credits) Restructuring, Settlement And Impairment Provisions, Excluding Goodwill Impairment Loss Restructuring, Settlement And Impairment Provisions, Excluding Goodwill Impairment Loss Total liability derivatives Derivative Liability, Subject to Master Netting Arrangement, before Offset Gross profit Gross Profit Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] South America, Middle East and other South America, Middle East, and Other [Member] South America, Middle East, and Other [Member] Accrued liabilities and other Increase (Decrease) in Other Operating Assets and Liabilities, Net Revenue from international sales Revenue From International Sales Revenue From International Sales Award Timing Predetermined Award Timing Predetermined [Flag] Treasury Stock Treasury Stock, Common [Member] Foreign Credit Agreements Foreign Credit Agreements [Member] Foreign Credit Agreements Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Debt Debt and Lease Obligation Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Effective income tax rate Effective Income Tax Rate Reconciliation, Percent Share-based compensation Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Nickel and other raw material contracts Nickel And Other Raw Material Contracts [Member] Derivative instrument whose primary underlying risk is tied to commodity prices for Nickel and other raw materials Allegheny Technologies Inc Allegheny Technologies Inc [Member] Allegheny Technologies Inc [Member] All Executive Categories All Executive Categories [Member] Shanghai STAL Precision Stainless Steel Co Ltd Shanghai STAL Precision Stainless Steel Co Ltd [Member] -- None. No documentation exists for this element. -- ATI Inc. 4.875% Senior Notes due 2029 ATI 2029 Senior Notes [Member] ATI 2029 Senior Notes Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Cash and cash equivalents and other Cash, Cash Equivalents And Other Cash, Cash Equivalents And Other Preferred stock, par value (USD per share) Preferred Stock, Par or Stated Value Per Share Scenario [Domain] Scenario [Domain] Reclassification from long-term Contract with Customer, Liability, Current, Reclassification Contract with Customer, Liability, Current, Reclassification Postretirement benefit plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax [Abstract] Reclassified to accounts receivable Contract with Customer, Asset, Current, Reclassified to Receivable Contract with Customer, Asset, Current, Reclassified to Receivable Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Business Segments Segment Reporting [Text Block] Amount utilized to support the issuance of letters of credit Long-Term Line of Credit Minimum Minimum [Member] Recognized in current year Contract with Customer, Liability, Noncurrent, Period Increase Contract with Customer, Liability, Noncurrent, Period Increase Contract Liabilities - Short-term Change in Contract with Customer, Liability, Current [Roll Forward] Change in Contract with Customer, Liability, Current [Roll Forward] Expiration Date Trading Arrangement Expiration Date Hedging Relationship [Axis] Hedging Relationship [Axis] Schedule of Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Denominator: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Diluted net income attributable to ATI per common share (in dollars per share) Income (Loss) from Continuing Operations, Per Diluted Share Basic net income attributable to ATI per common share (in dollars per share) Income (Loss) from Continuing Operations, Per Basic Share AA&S Advanced Alloys & Solutions [Member] Advanced Alloys & Solutions [Member] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Cash and cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Total Equity Beginning balance Ending balance Equity, Including Portion Attributable to Noncontrolling Interest Construction/Mining Construction Mining Market [Member] Construction Mining Market [Member] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Disposal Group, Including Discontinued Operations [Line Items] Disposal Group, Including Discontinued Operations [Line Items] Nonoperating retirement benefit expense Total retirement benefit expense Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Derivative [Table] Derivative [Table] Other Income and Expenses [Abstract] Other Income and Expenses [Abstract] Financing Receivable, Allowance for Credit Loss [Table] Financing Receivable, Allowance for Credit Loss [Table] Common stock, authorized (in shares) Common Stock, Shares Authorized Numerator for basic income per common share – Earnings Per Share Reconciliation [Abstract] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Exercise Price Award Exercise Price Restructuring Cost [Table] Restructuring Cost [Table] Investing Activities: Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Schedule of Equity Method Investments [Line Items] Schedule of Equity Method Investments [Line Items] Authorized amount Share Repurchase Program, Authorized, Amount Segment Reporting [Line Items] Segment Reporting [Line Items] Document Quarterly Report Document Quarterly Report Credit Facility Credit Facility Domestic Bank Group $600 million asset-based credit facility [Member] Domestic Bank Group $600 million asset-based credit facility Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Segments [Domain] Segments [Domain] Proceeds from sale and collection of receivables Proceeds from Sale and Collection of Receivables Europe Europe [Member] Schedule of Company Assets By Country Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Geographical Area, Long-Lived Asset [Table Text Block] Debt Debt Disclosure [Text Block] Scenario [Axis] Scenario [Axis] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Debt issuance costs Debt issuance costs Debt Issuance Costs, Net Schedule of Restructuring Reserve Restructuring and Related Costs [Table Text Block] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Restructuring reserve Balance at December 28, 2025 Balance at June 28, 2026 Restructuring Reserve Equity Component [Domain] Equity Component [Domain] PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Secured Debt Secured Debt [Member] Foreign exchange contracts Foreign Exchange Contract [Member] Proceeds from previous sales and collection of receivables Proceeds From Previous Sales And Collection Of Receivables Proceeds From Previous Sales And Collection Of Receivables Buildings and leasehold improvements Buildings and Improvements, Gross Adoption Date Trading Arrangement Adoption Date Schedule of Other Income, Net Schedule of Other Nonoperating Income (Expense) [Table Text Block] Inventory Inventory, Policy [Policy Text Block] Interest rate Interest rate Debt Instrument, Interest Rate, Stated Percentage Total ATI stockholders’ equity Equity, Attributable to Parent Write-off of the related unamortized debt issuance costs Deferred Debt Issuance Cost, Writeoff Disposal Group, Disposed of by Sale, Not Discontinued Operations Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] Restructuring Type [Axis] Restructuring Type [Axis] ASSETS Assets [Abstract] Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Accounting Policies [Abstract] Accounting Policies [Abstract] Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Insider Trading Policies and Procedures [Line Items] Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Derivative Instruments Gain Loss [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Selling and administrative expenses Selling, General and Administrative Expense Type of Restructuring [Domain] Type of Restructuring [Domain] Interest expense, net Interest expense, net Interest Income (Expense), Nonoperating Letter of credit sub-facility Letter of Credit [Member] Long-Lived Assets [Line Items] Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Geographical Area, Revenue and Long-Lived Asset [Line Items] Gain (loss) on sale of accounts receivable Gain (Loss) on Sale of Accounts Receivable Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Construction in progress Construction in Progress, Gross Number of business segments Number of Operating Segments Entity Central Index Key Entity Central Index Key Accumulated depreciation and amortization Property, Plant, and Equipment, Accumulated Depreciation, Depletion, and Amortization Additional paid-in capital Additional Paid in Capital, Common Stock All Adjustments to Compensation All Adjustments to Compensation [Member] Corporate: Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] Automotive Automotive Market [Member] Automotive Market [Member] Equity: Equity [Abstract] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation OCI before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Income Statement [Abstract] Income Statement [Abstract] ATI Stockholders’ Equity: Equity, Attributable to Parent [Abstract] Document Transition Report Document Transition Report Ownership [Axis] Ownership [Axis] Total AOCI Attributable to Noncontrolling Interest [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Investment, Name Investment, Name [Axis] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Receivables purchase and financing agreement, period Receivables Purchase And Financing Agreement, Period Receivables Purchase And Financing Agreement, Period Start Up Costs And Transaction Costs Start Up Costs And Transaction Costs [Member] Start Up Costs And Transaction Costs Variable interest rate (percentage) Debt Instrument, Basis Spread on Variable Rate City Area Code City Area Code Variable Rate [Domain] Variable Rate [Domain] Share repurchase program, utilized, amount Share Repurchase Program, Utilized, Amount Share Repurchase Program, Utilized, Amount Prior service (cost) credit Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] All Individuals All Individuals [Member] Subsequent Events Subsequent Events [Text Block] Corporate Corporate Segment [Member] Corporate expenses Corporate Expenses corporate office expenses not allocable to business segments Debt Debt Instrument, Fair Value Disclosure Retirement Benefits Pension and Other Postretirement Plans, Policy [Policy Text Block] Pension liabilities Liability, Defined Benefit Pension Plan, Noncurrent Name Forgone Recovery, Individual Name Commitments And Contingencies Commitments and Contingencies, Policy [Policy Text Block] Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Statement [Line Items] Statement [Line Items] Total Assets Total assets Assets Schedule Of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan [Table] Accrual for environmental remediation obligations Accrual for Environmental Loss Contingencies Notional amount of nickel hedge (in pounds of nickel) Derivative, Nonmonetary Notional Amount, Mass Preferred stock, issued (in shares) Preferred Stock, Shares Issued Percent of revenue Revenue from Contract with Customer, Excluding Assessed Tax, Percentage of Revenue Revenue from Contract with Customer, Excluding Assessed Tax, Percentage of Revenue Discrete tax benefit (expense) Effective Income Tax Rate Reconciliation, Discrete Tax Benefit (Expense), Amount Effective Income Tax Rate Reconciliation, Discrete Tax Benefit (Expense), Amount Amortization of prior service cost (credit) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Operating Segments Operating Segments [Member] Receivables Facility Receivables Facility [Member] Receivables Facility Accrued environmental loss contingencies recorded in other current liabilities Accrued Environmental Loss Contingencies, Current PEO PEO [Member] Owned or controlled sites at which Company operations have been or plan to be discontinued Environmental Remediation Obligation For Owned or Controlled Sites With Closed Operations Recorded amount of the accrual for a material component of an environmental loss contingency related to owned or controlled sites at which company operations have been discontinued Other Other Market [Member] Other Market [Member] Measurement Basis [Axis] Measurement Basis [Axis] Other long-term liabilities Other Liabilities, Noncurrent Goodwill Goodwill Retirement Plan Type [Axis] Retirement Plan Type [Axis] Actuarial loss Post Retirement Benefit Plans Actuarial Loss [Abstract] Post Retirement Benefit Plans Actuarial Loss [Abstract] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Subsequent Event Type [Domain] Subsequent Event Type [Domain] Retained Earnings Retained Earnings [Member] Revolving credit facility Revolving Credit Facility [Member] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Actuarial losses Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Amount utilized Receivables Purchase And Financing Agreement, Amount Utilized Receivables Purchase And Financing Agreement, Amount Utilized Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Common stock, outstanding (in shares) Common Stock, Shares, Outstanding Award Timing Disclosures [Line Items] Currency translation adjustment Currency translation adjustment Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Schedule of Financial Instrument Fair Value Fair Value, by Balance Sheet Grouping [Table Text Block] Debt issuance costs Payments of Debt Issuance Costs Diluted net income attributable to ATI per common share (in dollars per share) Earnings Per Share, Diluted Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Other Contract with Customer, Liability, Noncurrent, Other Contract with Customer, Liability, Noncurrent, Other Entity Current Reporting Status Entity Current Reporting Status Cash provided by (used in) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Less: Income taxes on derivative transactions Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, Tax Cash Flow Hedging Cash Flow Hedging [Member] Currency translation adjustment Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest [Member] Ownership [Domain] Ownership [Domain] Adjustments to reconcile net income to net cash provided by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Write-offs and recoveries of uncollectible accounts Accounts Receivable, Allowance for Credit Loss, Writeoff Financing Activities: Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Sales Revenue Revenue from Contract with Customer, Excluding Assessed Tax Property, plant and equipment, net Total property, plant and equipment, net Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Employee stock plans Employee Stock Plans Net change in stockholders' equity due to employee stock plans Interest cost on benefits earned in prior years Defined Benefit Plan, Interest Cost Minimum prepayment increments allowed Debt Instrument, Prepayment Increments, Minimum Debt Instrument, Prepayment Increments, Minimum Assets Derivative Asset Inventories Inventory Disclosure [Text Block] Stock repurchased during period, value Stock Repurchased During Period, Value Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Increase (decrease) in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect, Discontinued Operation Underlying Securities Award Underlying Securities Amount Less: Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Percent of total Entity Wide Disclosure On Geographic Areas Total Assets Attributed To Individual Countries Percentage Amount -- None. No documentation exists for this element. -- Total Estimated Fair Value Estimate of Fair Value Measurement [Member] Revenue from Contracts with Customers Revenue from Contract with Customer [Text Block] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Precision forgings, castings and components Precision Forgings, Casting and Components [Member] Precision Forgings, Casting and Components [Member] Other Payment for (Proceeds from) Other Investing Activity Inventories Increase (Decrease) in Inventories Schedule Of Segment Reporting Information By Segment [Table] Segment Reporting [Table] Borrowings Receivables Purchase And Financing Agreement, Borrowings Receivables Purchase And Financing Agreement, Borrowings Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Axis] Schedule of Equity Method Investments [Table] Equity Method Investment [Table] Employee retention credits Employee Retention Credits Employee Retention Credits Prepaid expenses and other current assets Other Assets, Current Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Derivative, gain (loss) on derivative, net Derivative, Gain (Loss) on Derivative, Net Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Operating income Operating Income (Loss) Confirmed order backlog Revenue, Remaining Performance Obligation, Amount Current Fiscal Year End Date Current Fiscal Year End Date Net loss on sales of businesses Gain (Loss) on Disposition of Business Current year amounts reclassified to revenue Contract with Customer, Liability, Current, Revenue Recognized Contract with Customer, Liability, Current, Revenue Recognized Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Derivative [Line Items] Derivative [Line Items] Asia Asia [Member] Derivatives Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract] Transformation Costs Transformation Costs [Member] Transformation Costs Line of credit facility, accordion feature, increase limit Line of Credit Facility, Accordion Feature, Increase Limit Line of Credit Facility, Accordion Feature, Increase Limit Stock repurchased during period, shares (in shares) Stock Repurchased During Period, Shares Restructuring Reserve [Roll Forward] Restructuring Reserve [Roll Forward] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Entity File Number Entity File Number Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Service cost - benefits earned during the year Defined Benefit Plan, Service Cost Equipment Machinery and Equipment, Gross Reclassification to net income of net realized (gain) loss Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax Asset impairment costs Asset, Impairment Loss Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Restructuring and other charges Restructuring and other charges Restructuring Costs Common stock, issued (in shares) Common Stock, Shares, Issued Measure: Measure [Axis] Joint Ventures Equity Method Investments and Joint Ventures Disclosure [Text Block] Finished goods Inventory, Finished Goods, Gross Treasury stock (in shares) Treasury Stock, Common, Shares Precision rolled strip products Precision rolled strip products [Member] Precision rolled strip products [Member] Accumulated Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Statements of Changes in Stockholders' Equity [Axis] Equity Components [Axis] Equity Components [Axis] Accounts receivable Increase (Decrease) in Accounts Receivable Product and Service [Domain] Product and Service [Domain] Contract costs for obtaining and fulfilling contracts Capitalized Contract Cost, Net Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Entity Interactive Data Current Entity Interactive Data Current Variable Rate [Axis] Variable Rate [Axis] Less: Net income attributable to noncontrolling interests Net Income (Loss) Attributable to Noncontrolling Interest Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Entity Small Business Entity Small Business Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Comprehensive income attributable to ATI Comprehensive Income (Loss), Net of Tax, Attributable to Parent Termination Date Trading Arrangement Termination Date Percentage of forecasted natural gas usage hedged for 2027 Percentage Of Forecasted Natural Gas Usage Hedged for Year 2 Aggregate notional amount of price risk derivatives for instruments with notional amounts expressed in percentage of estimated annual domestic (U.S.) usage Percentage of forecasted natural gas usage hedged for 2026 Percentage Of Forecasted Natural Gas Usage Hedged for Year 1 Percentage Of Forecasted Natural Gas Usage Hedged for Year 1 Accounts payable Increase (Decrease) in Accounts Payable, Trade Subsequent Events [Abstract] Debt covenant, undrawn availability requirement, percent of total available liquidity Debt Covenant, Undrawn Availability Requirement, Percent Of Total Available Liquidity Debt Covenant, Undrawn Availability Requirement, Percent Of Total Available Liquidity Divestitures Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Total Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Other current liabilities Other Liabilities, Current Other assets Other Assets, Noncurrent Document Type Document Type Accrued liabilities Accrued Liabilities, Current Fair Value as of Grant Date Award Grant Date Fair Value Debt instrument, redemption price, percentage Debt Instrument, Redemption Price, Percentage Accounting Policies Significant Accounting Policies [Text Block] Other Contract with Customer, Liability, Current, Other Contract with Customer, Liability, Current, Other Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Remaining authorized, amount Share Repurchase Program, Remaining Authorized, Amount Arrangement Duration Trading Arrangement Duration Subsequent Event [Line Items] Subsequent Event [Line Items] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Local Phone Number Local Phone Number Balance as of beginning of year Balance as of period end Contract with Customer, Liability, Noncurrent Other Postretirement Benefits Other Postretirement Benefits Plan [Member] Work-in-process Inventory, Work in Process, Gross Canada CANADA Supplemental Financial Statement Information Additional Financial Information Disclosure [Text Block] Consolidation Items [Axis] Consolidation Items [Axis] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Entity Registrant Name Entity Registrant Name Changes in operating assets and liabilities: Increase (Decrease) in Operating Assets [Abstract] Income before taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Property, plant and equipment, gross Property, Plant, and Equipment, before Accumulated Depreciation, Depletion, and Amortization Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Debt covenant, undrawn availability requirement, amount Debt Covenant, Undrawn Availability Requirement, Amount Debt Covenant, Undrawn Availability Requirement, Amount Executive Category: Executive Category [Axis] China CHINA Receivable Type [Axis] Receivable Type [Axis] LIABILITIES AND EQUITY Liabilities and Equity [Abstract] Inventories, net Total inventories, net Inventory, Net All Award Types Award Type [Domain] Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Amount of Gain (Loss) Reclassified from Accumulated OCI into Income Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax Total Other Markets Total Other Markets [Member] Total Other Markets Reclassification adjustment included in net income Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax Joint venture ownership percentage by unaffiliated entity Subsidiary, Ownership Percentage, Noncontrolling Owner PEO Name PEO Name Geographical [Axis] Geographical [Axis] Net borrowings under international credit facilities Proceeds from (Repayments of) Other Debt Other segment items Segment Reporting, Other Segment Item, Amount Average annual interest rate Line of Credit Facility, Interest Rate During Period Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Unrealized net change arising during the period Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax Entity Tax Identification Number Entity Tax Identification Number Accounts Receivable - Reserve for Doubtful Accounts Accounts Receivable, Allowance for Credit Loss [Roll Forward] Amount of Gain (Loss) Recognized in OCI on Derivatives Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax Purchase of treasury stock Treasury Stock, Value, Acquired, Cost Method Payments Payments for Restructuring Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation ATI Inc. 7.25% Senior Notes due 2030 ATI 2030 Senior Notes [Member] ATI 2030 Senior Notes Contract Assets Change in Contract with Customer, Current, Asset [Roll Forward] Change in Contract with Customer, Current, Asset [Roll Forward] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Base rate borrowings Base Rate [Member] Sales by U.S. operations to foreign countries Entity Wide Disclosure On Geographic Areas Revenue From External Foreign Customers Attributed To Entities Country Of Domicile total foreign country sales from domicile operations Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Schedule of Reclassifications Out of AOCI Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Non-NEOs Non-NEOs [Member] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Number of positions eliminated Restructuring and Related Cost, Number of Positions Eliminated Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Debt Instrument [Axis] Debt Instrument [Axis] Subsequent Event [Table] Subsequent Event [Table] Share-based compensation Share-Based Payment Arrangement, Noncash Expense Expected number of positions eliminated Restructuring and Related Cost, Expected Number of Positions Eliminated Percentage of estimated annual nickel requirements Percentage Of Raw Material Usage Hedged Aggregate notional amount of price risk derivatives for instruments with notional amounts expressed in percentage of annual estimated usage in production Land Land Accounts receivable, sale Accounts Receivable, Sale Disposal Group Name [Axis] Disposal Group Name [Axis] Recognized in current year Contract with Customer, Liability, Current, Period Increase Contract with Customer, Liability, Current, Period Increase Schedule of Debt Instruments Schedule of Debt [Table Text Block] Investment, Name [Domain] Investment, Name [Domain] Customer [Domain] Customer [Domain] Schedule of Inventories Schedule of Inventory, Current [Table Text Block] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Schedule Of Derivative Instruments In Statement Of Financial Position Fair Value Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Basic net income attributable to ATI per common share (in dollars per share) Earnings Per Share, Basic Minimum fixed charge coverage ratio allowed in event of default Debt Instrument, Covenant, Fixed Charge Coverage Ratio Debt Instrument, Covenant, Fixed Charge Coverage Ratio Purchase of treasury stock Payments for Repurchase of Common Stock Short-term contract liabilities Balance as of beginning of year Balance as of period end Contract with Customer, Liability, Current Total Current Assets Assets, Current Statistical Measurement [Axis] Statistical Measurement [Axis] Insider Trading Arrangements [Line Items] Schedule Of Derivative Instruments Gain Loss In Statement Of Financial Performance Derivative Instruments, Gain (Loss) [Table Text Block] Award Type Award Type [Axis] Net derivatives (loss) gain on hedge transactions Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Accounts receivable, net Accounts Receivable, after Allowance for Credit Loss, Current Retained earnings Retained Earnings (Accumulated Deficit) Product and Service [Axis] Product and Service [Axis] Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Pension Benefits Pension Plan [Member] Common Stock Common Stock [Member] Gain (loss) on sale of businesses Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal Federal Superfund and comparable state-managed sites Environmental Remediation Obligation Federal Superfund Sites Recorded amount of the accrual for a material component of an environmental loss contingency related to Federal Superfund and comparable state-managed sites Fair Value Measurement [Domain] Fair Value Measurement [Domain] Supplier finance program, obligation Supplier Finance Program, Obligation, Current Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Employee Severance Employee Severance [Member] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Cover [Abstract] Reclassifications Reclassification, Comparability Adjustment [Policy Text Block] Derivative Contract [Domain] Derivative Contract [Domain] (Gain) loss on asset sales and sales of businesses, net Gain (Loss) on asset and business sales, net Gain (Loss) on asset and business sales, net Foreign credit facilities Foreign Line of Credit [Member] Equity Awards Adjustments Equity Awards Adjustments [Member] Hedging Relationship [Domain] Hedging Relationship [Domain] Total Liabilities and Equity Liabilities and Equity Prior service cost Prior Service Cost [Abstract] Prior Service Cost [Abstract] Employee Stock Option Share-Based Payment Arrangement, Option [Member] Short-term debt and current portion of long-term debt Debt, Current Schedule Of Long Lived Assets [Table] Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Geographical Area, Revenue and Long-Lived Asset [Table] Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Noncontrolling interests Equity, Attributable to Noncontrolling Interest Finance leases and other Notes Payable, Other Payables [Member] Segments [Axis] Segments [Axis] Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Proceeds from issuance of senior notes Proceeds from Issuance of Senior Long-Term Debt Additions Restructuring charges Restructuring Charges Related Party [Domain] Related and Nonrelated Parties [Domain] Schedule of Accounts Receivable - Reserve for Doubtful Accounts Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Inventory Disclosure [Abstract] Inventory Disclosure [Abstract] Tabular List, Table Tabular List [Table Text Block] Formerly owned or operated sites Environmental Remediation Obligation For Formerly Owned Sites Recorded amount of the accrual for a material component of an environmental loss contingency related to formerly owned or operated sites Line of credit facility, increase to maximum borrowing capacity Line of Credit Facility, Increase to Maximum Borrowing Capacity Line of Credit Facility, Increase to Maximum Borrowing Capacity Other income, net Total other income, net Other Nonoperating Income (Expense) Closed operations and other income Other Operating Income (Expense), Net Name Awards Close in Time to MNPI Disclosures, Individual Name Average borrowings during period Line of Credit Facility, Average Outstanding Amount Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Nickel-based alloys and specialty alloys Nickel-based Alloys and Specialty Alloys [Member] Nickel-based Alloys and Specialty Alloys [Member] Schedule of Defined Benefit Plans Schedule of Defined Benefit Plans Disclosures [Table Text Block] Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Derivatives Fair Value [Line Items] Derivatives, Fair Value [Line Items] Raw materials and supplies Inventory, Raw Materials and Supplies, Gross Customer programs Customer programs [Member] Customer programs Recognized in current year Contract with Customer, Asset, Current, Period Increase Contract with Customer, Asset, Current, Period Increase Shares repurchased for income tax withholding on share-based compensation and other Payments for Repurchase of Other Equity Preferred stock, par value $0.10: authorized-50,000,000 shares; issued-none Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Amounts reclassified from AOCI Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Hedging Designation [Domain] Hedging Designation [Domain] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Purchases of property, plant and equipment Payments to Acquire Productive Assets Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Debt extinguishment costs Extinguishment of Debt, Amount Allocated corporate overhead Allocated Corporate Overhead Allocated Corporate Overhead Hedging Designation [Axis] Hedging Designation [Axis] Foreign exchange forward Foreign Exchange Forward [Member] Segment Reporting, Reconciliation of Profit (Loss) by Segment to Consolidated [Abstract] Segment Reporting, Reconciliation of Profit (Loss) by Segment to Consolidated [Abstract] Credit Facility [Axis] Credit Facility [Axis] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Long-term debt Long-Term Debt and Lease Obligation Discontinued Operations and Disposal Groups [Abstract] Name Measure Name Titanium and titanium-based alloys Titanium and Titanium-based Alloys [Member] Titanium and Titanium-based Alloys [Member] Income Taxes Income Tax Disclosure [Text Block] Accounts payable Accounts Payable, Current Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Site contingency, disbursement period, estimate, accrued Site Contingency, Disbursement Period, Estimate, Accrued Debt Instrument [Line Items] Debt Instrument [Line Items] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Defense Defense [Member] Defense Retirement Plan Type [Domain] Retirement Plan Type [Domain] Additional Paid-In Capital Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category Name Trading Arrangement, Individual Name Cash and cash equivalents Cash and Cash Equivalent Deferred Tax Asset Valuation Allowance Accumulated Deferred Tax Asset Valuation Allowance, Attributable To Noncontrolling Interest [Member] Accumulated Deferred Tax Asset Valuation Allowance, Attributable To Noncontrolling Interest Deferred Tax Asset Valuation Allowance Accumulated Deferred Tax Asset Valuation Allowance, Attributable To Parent [Member] Accumulated Deferred Tax Asset Valuation Allowance [Member] Company Selected Measure Amount Company Selected Measure Amount Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Depreciation and amortization Depreciation & amortization Depreciation, Depletion and Amortization Total inventories at current cost Inventory, Gross Secured Overnight Financing Rate (SOFR) Secured Overnight Financing Rate (SOFR) [Member] Net income Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest ATI Inc. 5.875% Senior Notes due 2027 ATI Inc. 5.875% Senior Notes due 2027 ATI 2027 Senior Notes [Member] ATI 2027 Notes Other Other Countries [Member] -- None. No documentation exists for this element. -- Contract with Customer, Sales Channel [Domain] Contract with Customer, Sales Channel [Domain] Current Assets: Assets, Current [Abstract] Liabilities Derivative Liability Contract Liabilities - Long-term Change in Contract with Customer, Liability, Noncurrent [Roll Forward] Change in Contract with Customer, Liability, Noncurrent [Roll Forward] Non-PEO NEO Non-PEO NEO [Member] Amortization of net actuarial loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Entity Address, Address Line One Entity Address, Address Line One Income tax provision Income tax expense (benefit) Income Tax Expense (Benefit) Conventional Energy Conventional energy [Member] Conventional energy Cost of sales Cost of sales Cost of Product and Service Sold Award Timing Method Award Timing Method [Text Block] Forecast Forecast [Member] Zirconium and related alloys Zirconium and Related Alloys [Member] Zirconium and Related Alloys [Member] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Deferred Taxes Deferred Tax Assets, Net of Valuation Allowance Amortization of net actuarial loss Defined Benefit Plan, Amortization of Gain (Loss) Net income attributable to ATI Net income attributable to ATI Net Income (Loss) Attributable to Parent Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Remaining expected consideration expected to be collected Disposal Group, Including Discontinued Operation, Remaining Expected Consideration To Be Collected Disposal Group, Including Discontinued Operation, Remaining Expected Consideration To Be Collected Schedule of Basic and Diluted Income Per Common Share Schedule of Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Table Text Block] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Unamortized debt issuance expense Unamortized Debt Issuance Expense Balance Sheet Location [Axis] Location, Statement of Financial Position, Balance [Axis] Bridge Loan Bridge Loan [Member] Statement Table [Table] Statement [Table] Proceeds from disposal of property, plant and equipment Proceeds from Sale of Property, Plant, and Equipment Restatement Determination Date: Restatement Determination Date [Axis] China Baowu Steel Group Corporation Limited China Baowu Steel Group Corporation Limited [Member] China Baowu Steel Group Corporation Limited [Member] Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Contract Assets and Liabilities Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] Total asset derivatives Derivative Asset, Subject to Master Netting Arrangement, before Offset Loss contingency, new claims filed, number Loss Contingency, New Claims Filed, Number Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Percent Maximum Borrowing Capacity Debt Instrument, Covenant, Fixed Coverage Ratio Restriction, Percent Maximum Borrowing Capacity Accrued postretirement benefits Liability, Other Postretirement Defined Benefit Plan, Noncurrent Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract]