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Receivable, Allowance for Credit Loss, Current Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Income tax refunds Proceeds from Income Tax Refunds PSU's Performance Shares [Member] Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Line of credit, amortization of principal amount, year one, percent Line of Credit, Amortization of Principal Amount, Year One, Percent Line of Credit, Amortization of Principal Amount, Year One, Percent Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Global Development Services Global Development Services [Member] Global Development Services Business Combination [Domain] Business Combination [Domain] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Securities excluded from the calculation of dilutive potential common shares (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount RSU's Restricted Stock Units (RSUs) [Member] Summary of changes in the carrying value of goodwill Goodwill [Table Text Block] Gross amount of goodwill Goodwill, Gross Entity Central Index Key Entity Central Index Key Summary of estimated contingent earn out liabilities Schedule of Estimated Contingent Earn Out Liabilities [Table Text Block] Schedule of Estimated Contingent Earn Out Liabilities Accumulated depreciation Property, Plant, and Equipment, Accumulated Depreciation, Depletion, and Amortization Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component All Adjustments to Compensation All Adjustments to Compensation [Member] Corporate Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] Business Combination [Axis] Business Combination [Axis] Equity [Abstract] 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] Net cash provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Settlement amount Payments for Legal Settlements Other comprehensive loss before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Income Statement [Abstract] Income Statement [Abstract] Shares issued for Employee Stock Purchase Plan (in shares) Stock Issued During Period, Shares, Employee Stock Purchase Plans Document Transition Report Document Transition Report Cash paid during the period for: Cash Paid During the Period [Abstract] Deferred tax assets Deferred Income Tax Assets, Net Intersegment 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Name Forgone Recovery, Individual Name Non-current contingent earn-out liabilities Business Combination, Contingent Consideration, Liability, Noncurrent Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Equity interests issued and issuable Business Combination, Consideration Transferred, Equity Interest Statement [Line Items] Statement [Line Items] Total assets Assets FCA Claims FCA Claims [Member] FCA Claims 2028 Senior Notes 2028 Senior Notes [Member] 2028 Senior Notes 5Y Term Loan Facility 5Y Term Loan Facility [Member] 5Y Term Loan Facility Effective Income Tax Rate Reconciliation [Line Items] Effective Income Tax Rate Reconciliation [Line Items] Contingent Consideration Contingent Consideration [Member] Contingent Consideration Accounts Receivable, Net Accounts Receivable [Policy Text Block] Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Convertible notes Convertible Debt [Member] Percentage of shares that ultimately vest depending on growth in diluted earnings per share Share Based Compensation Arrangement Percentage Of Awarded Shares That Ultimately Vests1 Represents the percentage of performance-based shares that ultimately vests, depending upon the fiscal year growth in the diluted earnings per share. Percentage of shares that ultimately vest depending on the shareholder return relative to peer group of companies over vesting period Share Based Compensation Arrangement Percentage Of Awarded Shares That Ultimately Vests2 Represents the percentage of performance-based shares that ultimately vests, depending upon the fiscal year shareholder return relative to peer group of companies over the vesting period of the reporting entity. Beyond Lessee, Operating Lease, Liability, To Be Paid, Due After Year Four Lessee, Operating Lease, Liability, To Be Paid, Due After Year Four Operating Segments Operating Segments [Member] Interest expense, net Interest Expense, Nonoperating Shares issued for Employee Stock Purchase Plan Stock Issued During Period, Value, Employee Stock Purchase Plan Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Proceeds from convertible debt Proceeds from Convertible Debt PEO PEO [Member] Beginning balance (in shares) Ending balance (in shares) Shares, Outstanding Stock-based compensation expense Share-Based Payment Arrangement, Expense International International [Member] Represents the international client sector. Stock repurchases (in shares) Stock Repurchased and Retired During Period, Shares Other non-current liabilities Other Liabilities, Noncurrent Goodwill Balance at beginning of the period Balance at end of the period Balance Goodwill Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Acquisition activities Goodwill, Acquired During Period Stock options exercised (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Ratio of consolidated fixed charge coverage ratio Ratio Of Consolidated Fixed Charge Coverage Ratio Represents the consolidated fixed charge coverage ratio as defined by the loan agreement. 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(Loss) [Table] Entity Current Reporting Status Entity Current Reporting Status Net cash used in financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Total Shareholder Return Amount Total Shareholder Return Amount Prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Litigation Case [Domain] Litigation Case [Domain] Amortization expense Amortization expense for intangible assets Intangible Asset, Finite-Lived, Amortization Expense Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Stockholders' Equity Note [Abstract] Stockholders' Equity Note [Abstract] Intangible Asset, Finite-Lived [Line Items] Intangible Asset, Finite-Lived [Line Items] Other Credit Facilities Other Credit Facilities [Member] Other Credit Facilities Cash flows from financing activities: Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Revenue Total consolidated revenue Revenue from Contract with Customer, Excluding Assessed Tax Property and equipment, net Property and equipment, net Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization Excise taxes accrued but not paid Excise Taxes Accrued But Not Paid Excise Taxes Accrued But Not Paid CERCLA Claims CERCLA Claims [Member] CERCLA Claims Assets held-for-sale Disposal Group, Including Discontinued Operation, Assets, Current Gain on cash surrender value of life insurance policies Gain On Cash Surrender Value Of Life Insurance Gain On Cash Surrender Value Of Life Insurance Total cost Stock Repurchased During Period, Value Intangible Asset, Finite-Lived, Class [Axis] Intangible Asset, Finite-Lived, Class [Axis] Stock options exercised Stock Issued During Period, Value, Stock Options Exercised Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Goodwill and Intangible Assets Intangible Asset and Goodwill [Text Block] Underlying Securities Award Underlying Securities Amount Number of contracts cancelled Number Of Contracts Cancelled Number Of Contracts Cancelled Less: Comprehensive income attributable to noncontrolling interests, net of tax Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Revenue and Contract Balances Revenue from Contract with Customer [Text Block] Operating leases Operating Lease, Weighted Average Discount Rate, Percent Variable Rate Component [Domain] Variable Rate Component [Domain] Variable Rate Component [Domain] Cash holdback related to tax reserve Business Combination, Consideration Transferred, Cash Holdback Related To Tax Reserve Business Combination, Consideration Transferred, Cash Holdback Related To Tax Reserve Segment Reporting [Table] Segment Reporting [Table] Share repurchase program, excise tax Share Repurchase Program, Excise Tax Schedule of Related Party Transactions, by Related Party [Table] Related Party Transaction [Table] Title of Individual [Axis] Title and Position [Axis] Beyond Finite-Lived Intangible Asset, Expected Amortization, after Year Four Finite-Lived Intangible Asset, Expected Amortization, after Year Four Summary of components of lease cost Lease, Cost [Table Text Block] Remaining performance obligation, termination notice period three Revenue, Remaining Performance Obligation, Termination Notice Period Three Revenue, Remaining Performance Obligation, Termination Notice Period Three Current contingent earn-out liabilities Business Combination, Contingent Consideration, Liability, Current Weighted average interest rate, at point in time Long-Term Debt, Weighted Average Interest Rate, at Point in Time Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Income from operations Segment operating income Operating Income (Loss) Summary of dividends declared and paid Dividends Declared [Table Text Block] Remaining unsatisfied performance obligation Revenue, Remaining Performance Obligation, Amount Current Fiscal Year End Date Current Fiscal Year End Date Gain on sale of divested business Gain on sale of divested business Gain (Loss) on Disposition of Business Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount U.S. state and local government State And Local Government [Member] Represents the state and local government client sector. 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liability Business Combination, Contingent Consideration, Liability Legal contingency costs Litigation Settlement, Fee Expense Operating lease cost Operating Lease, Cost Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover page. Cover [Abstract] Net tangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Tangible Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Tangible Assets Summary of acquired identifiable intangible assets with finite useful lives Schedule Of Gross And Accumulated Amounts For Acquired Finite Lived Intangible Assets By Major Class [Table Text Block] Tabular disclosure of the characteristics, including gross value, accumulated amortization amount, weighted average useful life, of finite-lived intangible assets acquired during the period by major class. A major class is composed of intangible assets that can be grouped together because they are similar, either by nature or by their use in the operations of the company. Interest expense Interest Expense, Debt, Excluding Amortization Period for billing and collecting unbilled receivables Unbilled Contracts Receivable Period for Billing and Collection Represents the period within which substantially all unbilled receivables are expected to be billed and collected. Equity Awards Adjustments Equity Awards Adjustments [Member] Amended Credit Agreement Amended Credit Agreement [Member] Amended Credit Agreement Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Summary of convertible debt Convertible Debt [Table Text Block] Number of businesses acquired Business Combination, Number of Businesses Acquired Total liabilities and stockholders' equity Liabilities and Equity Employee Stock Option Share-Based Payment Arrangement, Option [Member] Noncontrolling interests Equity, Attributable to Noncontrolling Interest Nondeductible expense, legal contingency charge, amount Effective Income Tax Rate Reconciliation, Nondeductible Expense, Legal Contingency Charge, Amount Effective Income Tax Rate Reconciliation, Nondeductible Expense, Legal Contingency Charge, Amount Segments [Axis] Segments [Axis] Other liabilities Increase (Decrease) in Other Operating Liabilities Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Related Party [Domain] Related and Nonrelated Parties [Domain] Summary of components of net accounts receivable Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Fourth Amended Restated Credit Agreement Fourth Amended Restated Credit Agreement [Member] Fourth Amended Restated Credit Agreement Cash settled on contingent earn-out liabilities Increase (Decrease) In Cash Settled, Contingent Earn-Out Liability Increase (Decrease) In Cash Settled, Contingent Earn-Out Liability Tabular List, Table Tabular List [Table Text Block] Other non-operating income Other Nonoperating Income (Expense) Long-Term Debt Long-Term Debt [Text Block] Total Tetra Tech Equity Parent [Member] 2029 Intangible Asset, Finite-Lived, Estimated Amortization Expense, Year Three Shares issuable assuming conversion of convertible notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities 2026 (remaining) Intangible Asset, Finite-Lived, Estimated Amortization Expense, Remainder of Fiscal Year Name Awards Close in Time to MNPI Disclosures, Individual Name Equity: Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Carron + Walsh And Sage Group Holdings Carron + Walsh And Sage Group Holdings [Member] Carron + Walsh And Sage Group Holdings 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] Accounts receivable and contract assets Increase (Decrease) In Accounts Receivable And Contract With Customer, Asset Increase (Decrease) In Accounts Receivable And Contract With Customer, Asset Property, Plant and Equipment, Type [Domain] Long-Lived Asset, Class [Domain] Halvik Corp Halvik Corp [Member] Halvik Corp Fair Value by Liability Class [Domain] Fair Value by Liability Class [Domain] Preferred stock, authorized shares (in shares) Preferred Stock, Shares Authorized U.S. commercial Commercial [Member] Represents the commercial client sector. Property, Plant, and Equipment [Table] Property, Plant, and Equipment [Table] Preferred stock - authorized, 2,000 shares of $0.01 par value; no shares issued and outstanding at June 28, 2026 and September 28, 2025 Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Reclassification to earnings from sale of divested business Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Interest Interest Paid, Excluding Capitalized Interest, Operating Activity Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Credit Facility [Axis] Credit Facility [Axis] Reconciliation of profit (segment operating income): Segment Reporting, Reconciliation of Profit (Loss) by Segment to Consolidated [Abstract] Significant unobservable input, earn-out period Business Acquisition, Earn Out Period for Operating Income Projections Used in Fair Value Measurement Represents the earn out period for operating income projection used for fair value measurement. Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Name Measure Name Income Taxes Income Tax Disclosure [Text Block] Accounts payable Accounts Payable, Current Pay vs Performance Disclosure [Line Items] Other non-cash items Other Noncash Income (Expense) Debt Instrument [Line Items] Debt Instrument [Line Items] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Total purchase price Business Combination, Consideration Transferred Additional Paid-in Capital Additional Paid-in Capital [Member] Entity Filer Category Entity Filer Category 3Y Term Loan Facility 3Y Term Loan Facility [Member] 3Y Term Loan Facility Non-employee director Director [Member] Impairment of goodwill Impairment of goodwill Goodwill, Impairment Loss Name Trading Arrangement, Individual Name Cash and cash equivalents Cash and Cash Equivalent Company Selected Measure Amount Company Selected Measure Amount Line of credit, amortization of principal amount, year five, percent Line of Credit, Amortization of Principal Amount, Year Five, Percent Line of Credit, Amortization of Principal Amount, Year Five, Percent Depreciation and amortization Depreciation, Depletion and Amortization Secured Overnight Financing Rate (SOFR) Secured Overnight Financing Rate (SOFR) [Member] Number of qui tam actions Loss Contingency, Number Of Actions Loss Contingency, Number Of Actions Net income Net income Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Common stock, reported sales price (in dollars per share) Common Stock, Reported Sales Price Common Stock, Reported Sales Price Current assets: Assets, Current [Abstract] Operating leases Operating Lease, Weighted Average Remaining Lease Term Non-PEO NEO Non-PEO NEO [Member] Entity Address, Address Line One Entity Address, Address Line One Right-of-use assets obtained in exchange for new operating lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Fair value of awards granted (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Income tax expense Income Tax Expense (Benefit) GSG Government Services Group [Member] Government Services Group [Member] 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, Noncash Item [Abstract] Subcontractor costs Total consolidated subcontractor costs Cost of Product and Service Sold Award Timing Method Award Timing Method [Text Block] Net increase in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Including Exchange Rate Effect and Discontinued Operation Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Interest accretion expense Accretion Expense Total potential maximum outstanding Business Combination, Contingent Consideration, Range of Outcomes, Maximum, Amount Net favorable operating income adjustments Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Change in Measure of Progress Business combination, recognized liability assumed, deferred tax liability Business Combination, Recognized Liability Assumed, Deferred Tax Liability Time-and-materials Time-and-Materials Contract [Member] Recent Accounting Pronouncements Accounting Standards Update and Change in Accounting Principle [Text Block] Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Swingline Loan Bridge Loan [Member] Statement [Table] Statement [Table] Restatement Determination Date: Restatement Determination Date [Axis] Stock options exercised Proceeds from Stock Options Exercised Summary of reclassifications out of accumulated other comprehensive income Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Contract Retentions Contract Retentions [Member] Contract Retentions [Member] Summary of remaining performance obligation, expected timing Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block] Amount awarded to other party Litigation Settlement, Amount Awarded to Other Party Goodwill reallocation Goodwill, Transfers Summary of net contract assets/liabilities Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block] Fed Funds Rate Fed Funds Effective Rate Overnight Index Swap Rate [Member] Effect of foreign currency exchange rate changes Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss) Contract liability revenue recognized during the period Contract with Customer, Liability, Revenue Recognized, Including Opening Balance Contract with Customer, Liability, Revenue Recognized, Including Opening Balance Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Contingent consideration – fair value adjustments Fair value adjustments to contingent consideration Fair Value Adjustments To Contingent Consideration Fair Value Adjustments To Contingent Consideration Non-cash financing activities: Other Noncash Investing and Financing Items [Abstract] Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block]