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Stock-Based Plans and Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Montrose 2017 Stock Incentive Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |
Options to Purchase Common Stock Outstanding Beginning Balance | 2,345,207 | 2,516,272 | 2,516,272 |
Options to Purchase Common Stock Forfeited/cancelled | (40,368) | (71,915) |  |
Options to Purchase Common Stock Expired | (65,287) | (20,510) |  |
Options to Purchase Common Stock Exercised | (16,804) | (51,524) |  |
Options to Purchase Common Stock Outstanding Ending Balance | 2,222,748 | 2,372,323 | 2,345,207 | 2,516,272
Options to Purchase Common Stock Exercisable | 1,995,520 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] |  |  |  |
Weighted-Average Exercise Price per Share Outstanding Beginning Balance | $ 30.62 | $ 30.92 | $ 30.92 |
Weighted-Average Exercise Price per Share Forfeited/cancelled | 41.5 | 38.64 |  |
Weighted-Average Exercise Price per Share Expired | 36.89 | 45.57 |  |
Weighted-Average Exercise Price per Share Exercised | 15.45 | 26.21 |  |
Weighted-Average Exercise Price per Share Outstanding Ending Balance | 30.35 | 30.66 | 30.62 | $ 30.92
Weighted-Average Exercise Price per Share Exercisable | 29.11 |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance | 16.32 | 15.95 | 15.95 |
Weighted Average Grant Date Fair Value per Share Outstanding Ending Balance | $ 16.32 | $ 16.3 | $ 16.32 | $ 15.95
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |
Weighted Average Remaining Contract Life (in Years) Outstanding | 5 years 2 months 12 days | 6 years 3 months 18 days | 6 years | 7 years
Weighted Average Remaining Contract Life (in Years) Exercisable | 5 years 1 month 6 days |  |  |
Aggregate Intrinsic Value of In-The-Money Options Exercised | $ 164 | $ 732 |  |
Aggregate Intrinsic Value of In-The-Money Options Outstanding | 8,241 | $ 7,517 | $ 776 | $ 13,825
Aggregate Intrinsic Value of In-The-Money Options Exercisable | $ 8,241 |  |  |
Montrose Amended and Restated 2013 Stock Option Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |
Options to Purchase Common Stock Outstanding Beginning Balance | 680,889 | 792,191 | 792,191 |
Options to Purchase Common Stock Expired | (1,000) |  |  |
Options to Purchase Common Stock Exercised | (18,095) | (105,453) |  |
Options to Purchase Common Stock Outstanding Ending Balance | 661,794 | 686,738 | 680,889 | 792,191
Options to Purchase Common Stock Exercisable | 661,794 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] |  |  |  |
Weighted-Average Exercise Price per Share Outstanding Beginning Balance | $ 6.49 | $ 6.40 | $ 6.40 |
Weighted-Average Exercise Price per Share Expired | 6.03 |  |  |
Weighted-Average Exercise Price per Share Exercised | 8.02 | 5.86 |  |
Weighted-Average Exercise Price per Share Outstanding Ending Balance | 6.46 | 6.49 | 6.49 | $ 6.40
Weighted-Average Exercise Price per Share Exercisable | 6.45 |  |  |
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] |  |  |  |
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance | 2.51 | 2.16 | 2.16 |
Weighted Average Grant Date Fair Value per Share Outstanding Ending Balance | $ 2.51 | $ 2.49 | $ 2.51 | $ 2.16
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] |  |  |  |
Weighted Average Remaining Contract Life (in Years) Outstanding | 9 months 18 days | 1 year 9 months 18 days | 1 year 6 months | 2 years 4 months 24 days
Weighted Average Remaining Contract Life (in Years) Exercisable | 9 months 18 days |  |  |
Aggregate Intrinsic Value of In-The-Money Options Exercised | $ 309 |  |  |
Aggregate Intrinsic Value of In-The-Money Options Outstanding | 10,351 | $ 13,606 | $ 8,211 | $ 20,380
Aggregate Intrinsic Value of In-The-Money Options Exercisable | $ 10,351 |  |  |

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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(04) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(03) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Number of options outstanding, including both vested and non-vested options. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Weighted average price at which option holders acquired shares when converting their stock options into shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(02) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(04) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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