Significant Accounting Policies (Policies)	12 Months Ended
Significant Accounting Policies (Policies)	Dec. 31, 2025
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation—The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (GAAP). Intercompany balances and transactions are eliminated.
Use of Estimates	Use of Estimates—The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Significant estimates inherent in the preparation of the accompanying consolidated financial statements include, but are not limited to, management’s forecasts of future cash flows used as a basis to assess recoverability of goodwill and long-lived assets, the allocation of purchase price to tangible and intangible assets, allowances for doubtful accounts, the estimated useful lives over which property and equipment is depreciated and intangible assets are amortized, subsequent measurement of goodwill, the fair value of contingent consideration payables, the fair value of embedded derivatives, equity-based compensation expense and deferred taxes. These estimates could materially differ from actual results.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash—The Company maintains its cash in bank deposit accounts, which at times may exceed federally insured limits. The Company considers cash deposits in banks as cash with original maturities at purchase of three months or less as cash equivalents. Cash, long-term debt and financial instruments subject the Company to concentrations of credit risk. To minimize the risk of credit loss, these financial instruments are primarily held with large, reputable financial institutions. The Company has not experienced losses in such accounts and believes it is not exposed to any significant credit risk associated with these accounts. Cash that is restricted as to withdrawal or use under the terms of certain contractual agreements is recorded in restricted cash in the Company’s consolidated statements of financial position. The Company's restricted cash balances were $0.2 million and $1.5 million as of December 31, 2025 and 2024, respectively.
Accounts Receivables-Net	Accounts Receivables-Net—Accounts receivable are presented in the consolidated statements of financial position, net of an allowance for doubtful accounts. The allowance for doubtful accounts is established at the origination of an account in accordance with Accounting Standard Update (ASU) 2016-13, Financial Instruments—Credit Losses (Topic 326). Accounting Standards Codification (ASC) 326 requires the Company to estimate the lifetime expected credit losses on such instruments and to record an allowance to offset the receivables.
Financial Instruments	Financial Instruments—The Financial Accounting Standards Board (FASB) ASC 820, Fair Value Measurements and Disclosures, defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date. ASC 820 establishes a three-level fair value hierarchy that prioritizes the inputs and minimizes the use of unobservable inputs. The three levels of inputs used to measure fair values are as follows: Level 1—Quoted prices in active markets for identical assets or liabilities. Level 2—Observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; or other inputs that are observable or can be corroborated by observable market data. Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. This includes certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs. The inputs to the determination of fair value are based upon the best information in the circumstances and may require significant management judgment or estimation. The Company considers the carrying values of cash, cash equivalents, restricted cash, accounts receivable, accounts payable, and accrued expenses to approximate fair value for these financial instruments due to the short maturities of these instruments. The Company’s interest rate swap, embedded derivatives, and any acquisition’s contingent consideration are carried at fair value and determined according to the fair value hierarchy above. The Company’s variable rate borrowings under its Credit Facility (Note 13) is tied to market indices and, thus, approximate fair value. The estimated fair value of the long-term debt under the credit facility is based on borrowing rates currently available to the Company for loans with similar terms and remaining maturities.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets—Certain events or changes in circumstances may indicate that the recoverability of the carrying amount of long lived assets should be assessed. When such events or changes in circumstances are present, the Company estimates the future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition. If the sum of the expected undiscounted future cash flows is less than the carrying amount, the Company recognizes an impairment based on the fair value of such assets.
Acquisitions	Acquisitions—The Company first assesses whether the acquisition represents a purchase of assets or a business. If the transaction is a business acquisition, the Company accounts for the acquisition using business combination accounting, which requires that assets acquired and liabilities assumed be recognized at fair value as of the acquisition date. The purchase price of acquisitions is allocated to the tangible and identifiable intangible assets acquired and liabilities assumed based on estimated fair values, and any excess purchase price over the identifiable assets acquired and liabilities assumed is recorded as goodwill. Goodwill represents the premium the Company pays over the fair value of the net tangible and intangible assets acquired. The Company may use independent valuation specialists to assist in determining the estimated fair values of assets acquired and liabilities assumed, which could require certain significant management assumptions and estimates. Transaction costs associated with acquisitions of businesses are expensed as they are incurred.
Business Acquisition Contingencies	Business Acquisition Contingencies—Some of the Company’s acquisition agreements include contingent consideration arrangements, which are generally based on the achievement of future performance thresholds. For each transaction, the Company estimates the fair value of contingent consideration payments as part of the initial purchase price and record the estimated fair value of contingent consideration as a liability. Subsequent changes in the fair value of contingent consideration are recognized as a gain or loss in the consolidated statements of operations. Payments of contingent consideration are reflected in financing activities in the consolidated statements of cash flows to the extent included as part of the initial purchase price, or in operating activities if the payment exceeds the amount included in the initial purchase price.
Goodwill	Goodwill—Goodwill is not amortized but instead qualitatively or quantitatively tested for impairment at least annually. Should an event or circumstances indicate that a reduction in fair value of the reporting unit may have occurred during the year, goodwill would also be tested at such occasion. The Company performs its goodwill test at the reporting unit level. If necessary, the goodwill quantitative impairment test is performed on October 1st every year. The Company uses a two-step process to assess the realizability of goodwill. The first step (generally referred to as a "step 0" analysis) is a qualitative assessment that analyzes current economic indicators associated with a particular reporting unit. For example, the Company analyzes changes in economic, market and industry conditions, business strategy, cost factors, and financial performance, among others, to determine if there are indicators of a significant decline in the fair value of a particular reporting unit. If the qualitative assessment indicates a stable or improved fair value, no further testing is required. If a qualitative assessment indicates it is more likely than not that the fair value of a reporting unit is less than its carrying amount, the Company will proceed to the quantitative second step (generally referred to as a "step 1" analysis). Step 1 of the quantitative test requires comparison of the carrying value of each of the reporting units to the respective fair value, calculated based on weighted income and market-based approaches. If the carrying value of the reporting unit is less than the fair value, no impairment exists. Otherwise, the Company would recognize an impairment charge for the amount by which the carrying amount of a reporting unit exceeds its fair value up to the amount of goodwill allocated to that reporting unit. The Company's annual assessment of goodwill has historically been completed as of the beginning of the fourth quarter each year. The Company performed its 2025 and 2024 annual assessments as of October 1 of each year and determined that no impairment existed as the estimated fair value of each of the Company's reporting units was in excess of its respective carrying value. Also, no triggering events or changes in circumstances occurred during the period October 1, 2025 through December 31, 2025 that warranted retesting goodwill for impairment.
Embedded Derivatives	Embedded Derivatives—Embedded derivatives that are required to be bifurcated from the underlying host instrument are accounted for and valued as a separate financial instrument. These embedded derivatives are bifurcated, accounted for at their estimated fair value, which is based on certain estimates and assumptions, and presented separately on the consolidated statements of financial position. Our valuation of embedded derivatives follows the With and Without method of the income approach, where the value of the derivative is derived by comparing projected cash flows with and without the embedded feature. The discount rate reflects the level of risk associated with these cash flows and is determined based on and evaluation of the Company’s credit risk and market required yields for comparable securities with similar credit risk. To derive a credit rating indication, the Company utilizes the Synthetic Credit Rating Model, and the recovery rate method is employed to establish the Company’s discount rate. Changes in fair value of the embedded derivatives are recognized as a component of other income/expense on the Company’s consolidated statements of operations (Note 16).
Foreign Currency	Foreign Currency—The Company historically has operations in the United States, Canada, Australia and Europe. In 2025, the Company exited all operations in Europe. The results of its non-U.S. dollar based functional currency operations are translated to U.S. dollars at the average exchange rates during the period. The Company’s assets and liabilities are translated using the exchange rate as of the date of the consolidated statement of financial position and equity is translated using historical rates. Adjustments resulting from the translation of the consolidated financial statements of the Company’s foreign functional currency subsidiaries into U.S. dollars are excluded from the determination of net income (loss) and instead are included in accumulated other comprehensive loss as a separate component of stockholders’ equity. Foreign exchange transaction gains and losses are included in selling, general and administrative expense on the consolidated statements of operations.
Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss—Accumulated other comprehensive loss, as presented on the consolidated statements of redeemable convertible and redeemable Series A-2 Preferred Stock and stockholders’ equity, consists of unrealized gains and losses on foreign currency translation. Comprehensive income (loss) is not included in the computation of income tax benefit.
Revenue Recognition	Revenue Recognition—Revenue is recognized in accordance with ASC Topic 606, Revenue from Contracts with Customers. The following is considered by the Company in the recognition of revenue under ASC 606: The Company’s services are performed under two general types of contracts (i) fixed-price and (ii) time-and-materials. Under fixed-price contracts, customers pay an agreed-upon amount for a specified scope of work agreed to in advance of the project. Under time-and-materials contracts, customers pay for the hours worked and resources used based on agreed-upon rates. Certain of the Company’s time-and-materials contracts are subject to maximum contract amounts. The duration of the Company’s contracts ranges from less than one month to over a year, depending on the scope of services provided. Payment terms are agreed upon at the time of contract approval and are typically net 30. Costs to obtain and fulfill contracts associated with system sales are expensed as a cost of revenue when the Company has fulfilled its performance obligations. The Company accounts for individual promises in contracts as separate performance obligations if the promises are distinct. The assessment requires judgment. The majority of the Company’s contracts have a single performance obligation as the promise to transfer the individual goods or services is not separately identifiable from other promises in the contracts and is, therefore, not distinct. Certain contracts in the Company’s Measurement and Analysis segment have multiple performance obligations, most commonly due to the contracts providing for multiple laboratory tests which are individual performance obligations. For the Measurement and Analysis contracts with multiple performance obligations, the Company allocates the transaction price to each performance obligation based on the relative standalone selling price of each performance obligation. The standalone selling price of each performance obligation is generally determined by the observable price of a service when sold separately. Fixed fee contracts—On the majority of fixed fee contracts, the Company recognizes revenue, over time, using either the proportion of actual costs incurred to the total costs expected to complete the contract performance obligation (cost to cost method), or the time-elapsed basis. The Company determined that the cost to cost method best represents the transfer of services as the proportion closely depicts the efforts or inputs completed towards the satisfaction of a fixed fee contract performance obligation. Under the time-elapsed basis, the arrangement is considered a single performance obligation comprised of a series of distinct services that are substantially the same and that have the same pattern of transfer (i.e. distinct days of service). The Company applies a time-based measure of progress to the total transaction price, which results in ratable recognition over the term of the contract. For a portion of the Company’s laboratory service contracts, revenue is recognized as performance obligations are satisfied over time, with recognition reflecting a series of distinct services using the output method. The Company determined that this method best represents the transfer of services as the customer obtains equal benefit from the service throughout the service period. There are inherent uncertainties in the estimation process for cost to cost contracts, as the estimation of total contract costs and estimates to complete is complex, subject to many variables, and requires judgment. It is possible that estimates of costs to complete a performance obligation will be revised in the near-term based on actual progress and costs incurred. These uncertainties primarily impact the Company’s contracts in the Remediation and Reuse segment. Time-and-materials contracts—Time-and-materials contracts contain variable consideration. However, these arrangements qualify for the “Right to Invoice” practical expedient. Under this practical expedient, the Company recognized revenue, over time, in the amount to which the Company has a right to invoice. In addition, the Company is not required to estimate such variable consideration upon inception of the contract and reassess the estimate each reporting period. The Company determined that this method best represents the transfer of services as, upon billing, the Company had a right to consideration from a customer in an amount that directly corresponded with the value to the customer of the Company’s performance completed to date.
Segment Reporting	Segment Reporting—Operating segments are components of an enterprise for which discrete financial reporting information is available and evaluated regularly by the Chief Operating Decision Maker (CODM) in deciding how to allocate resources and in assessing performance. The Company has identified its Chief Executive Officer as the CODM. The CODM views the Company’s operations and manages the businesses as three operating segments, which are also the Company’s reportable segments: (i) Assessment, Permitting and Response, (ii) Measurement and Analysis, and (iii) Remediation and Reuse.
Stock-Based Compensation	Stock-Based Compensation—The Company sponsors stock incentive plans that allow for issuance of employee stock options, restricted stock awards, restricted stock units and stock appreciation rights awards. There are certain awards that were issued to non-employees in exchange for their services and are accounted for under ASC 505, Equity-Based Payments to Non-Employees. ASC 505 requires that the fair value of the equity instruments issued to a non-employee be measured on the earlier of: (i) the performance commitment date or (ii) the date the services required under the arrangement have been completed. Certain of the performance based restricted stock units will only meet the requirements for establishing a grant date when the final calculated financial performance metrics and the amount of awards have been approved by the Company’s Board of Directors, which will then trigger the service inception date, the fair value of the awards, and the associated expense recognition period. The fair value of the remaining stock-based payment awards is expensed over the vesting period of each tranche on a straight-line basis. Any modification of an award that increases its fair value will require the Company to recognize additional expense. The fair value of stock options under its employee stock incentive plan are estimated as of the grant date using the Black-Scholes option valuation model, which is affected by its expected dividend yield, expected term and the expected share price volatility of its common shares over the expected term. No dividend rates are used in the calculation as these are not applicable to the Company. Forfeitures are recognized as incurred. Employee options are accounted for in accordance with the guidance set forth by ASC 718, Stock Based Compensation.
Income Taxes	Income Taxes—The Company accounts for income taxes under the asset and liability method, which requires recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in the period that includes the enacted date. A valuation allowance is recorded when it is more-likely-than-not some of the deferred tax assets may not be realized. Significant judgment is applied when assessing the need for a valuation allowance and the Company considers all available positive and negative evidence, including future taxable income, reversals of existing deferred tax assets and liabilities and ongoing prudent and feasible tax planning strategies in making such assessment. Should a change in circumstances lead to a change in judgment regarding the utilization of deferred tax assets in future years, the Company will adjust the related valuation allowance in the period such change in circumstances occurs. For acquired business entities, if the Company identifies changes to acquired deferred tax asset valuation allowances or liabilities related to uncertain tax positions during the measurement period, and they relate to new information obtained about facts and circumstances existing as of the acquisition date, those changes are considered a measurement period adjustment and the offset is recorded to goodwill. The Company records uncertain tax positions on the basis of the two-step process in which (i) it determines whether it is more-likely-than-not that the tax positions will be sustained on the basis of the technical merits of the position and (ii) for those tax positions that meet the more-likely-than-not recognition threshold, the Company would recognize the largest amount of tax benefit that is more than 50.0% likely to be realized upon ultimate settlement with the related tax authority. The Company classifies interest and penalties recognized on uncertain tax positions as a component of income tax expense.
Reclassifications	Reclassifications—Certain prior year amounts, which are not material, have been reclassified to conform to current year presentation in the consolidated statements of cash flows and notes to the consolidated financial statements.
Recently Adopted Accounting Pronouncements /Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements ASU 2023-05 —In August 2023, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2023-05 Business Combinations — Joint Venture Formations (Subtopic 805-60): Recognition and Initial Measurement, under which an entity that qualifies as either a joint venture or a corporate joint venture is required to apply a new basis of accounting upon the formation of the joint venture. Specifically, the ASU provides that a joint venture or a corporate joint venture must initially measure its assets and liabilities at fair value on the formation date. The amendments in ASU 2023-05 are effective for all joint ventures within the ASU’s scope that are formed on or after January 1, 2025. The Company adopted the standard on January 1, 2025. The adoption of the standard did not have a material impact on the Company's consolidated financial statements. ASU 2023-09 —In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures, which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. ASU 2023-09 is effective for the Company's fiscal year beginning after December 15, 2024 and is being applied using the retrospective approach. The Company adopted the standard on January 1, 2025. The adoption did not have an impact on the recognition or measurement of income taxes in the Company's consolidated financial statements, and only required additional disclosures. The Company updated its income tax disclosures in accordance with the guidance. Recently Issued Accounting Pronouncements Not Yet Adopted ASU 2024-03 —In November 2024, the FASB issued ASU 2024-03, Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses (ASU 2024-03), which is intended to improve the disclosures about a public business entity’s expenses and address requests from investors for more detailed information about the types of expenses in commonly presented expense captions (such as cost of sales; selling, general, and administrative expenses; and research and development). ASU 2024-03 is effective for the Company's fiscal year beginning January 1, 2027 and interim periods within fiscal years beginning after December 15, 2027, and allows the use of a prospective or retrospective approach. The Company plans to adopt the standard on January 1, 2027 and is currently evaluating the impact of the adoption of the standard on its consolidated financial statements. ASU 2025-06 —In September 2025, the FASB issued ASU 2025-06, Intangibles — Goodwill and Other — Internal-Use Software (Subtopic 350-40): Targeted Improvements to the Accounting for Internal-Use Software (ASU 2025-06), which removes all references to software development project stages, making the guidance neutral to different software development methodologies. Under the ASU, software capitalization will begin when management has authorized and committed to funding the software project and it is probable that the project will be completed and the software will be used as intended. ASU 2025-06 is effective for the Company's fiscal year beginning January 1, 2028, and interim periods within fiscal years beginning after December 15, 2027, and allows the use of a prospective, modified transition, or retrospective approach. Early adoption is permitted. The Company is currently evaluating the impact of the adoption of the standard on its consolidated financial statements.

Montrose Environmental Group, Inc. 10-K