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Stock-Based Plans and Compensation - Schedule of Common Stock Reserved for Future Issuance (Details) - shares | Sep. 30, 2025 | Jan. 31, 2025 | Sep. 30, 2024 | Jan. 31, 2024
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Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Common stock reserved for future issuance | 8,046,024 |  |  | 10,339,792 |  |
Montrose 2013 Stock Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Common stock reserved for future issuance | 661,794 |  |  | 686,738 |  |
Montrose 2017 Stock Incentive Plan |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |
Common stock reserved for future issuance | 7,384,230 | [1] | 1,372,373 | 9,653,054 | [1] | 1,207,563
[1] | In January 2025 and 2024, the Board of Directors ratified the addition of 1,372,373 and 1,207,563 shares of common stock, respectively, to the number of shares available for issuance under the 2017 Plan pursuant to the annual increase provision of such plan. Unless the Board of Directors determines otherwise, additional annual increases will be effective on each January 1, through January 1, 2027. The 2017 Plan permits the company to settle awards, if and when vested, in cash at its discretion. Pursuant to the terms of the 2017 Plan, the number of shares authorized for issuance thereunder will only be reduced with respect to shares of common stock actually issued upon exercise or settlement of an award. Shares of common stock subject to awards that have been canceled, expired, forfeited or otherwise not issued under an award and shares of common stock subject to awards settled in cash do not count as shares of common stock issued under the 2017 Plan. The Company expects to have sufficient shares available under the 2017 Plan to satisfy the future settlement of outstanding awards. Shares reserved for future issuance as of September 30, 2024 include 3,000,000 shares underlying the 3,000,000 2021 Performance SARs, which were cancelled effective as of December 31, 2024.

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