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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