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Stockholder's Equity (Deficit) - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
May 31, 2015
Class Of Stock [Line Items]        
Common stock, shares authorized 190,000,000   25,000,000  
Common stock, par value $ 0.000004   $ 0.000004  
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans $ 10.4 $ 6.7    
Unrecognized expense expected to be recognized period 2 years      
Montrose Amended & Restated 2017 Stock Incentive Plan        
Class Of Stock [Line Items]        
Number of restricted stock shares issued 33,229 30,000    
Montrose Amended & Restated 2017 Stock Incentive Plan | Restricted Stock        
Class Of Stock [Line Items]        
Restricted stock fair value market price per share $ 31.60 $ 24.00    
Shares of vested over period 1 year 3 years    
Restricted shares became fully vested and released as common stock 10,000 0    
Restricted shares outstanding 267,173 242,025    
Restricted shares forfeited 1,667 0    
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Board of Directors        
Class Of Stock [Line Items]        
Shares of vested over period 4 years      
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Executive Officers        
Class Of Stock [Line Items]        
Shares of vested over period 3 years      
Montrose Amended and Restated 2013 Stock Option Plan        
Class Of Stock [Line Items]        
Shares outstanding from exercised options 221,950   201,600  
Private Offering        
Class Of Stock [Line Items]        
Warrants to purchase common stock       116,350
Warrant exercise price per share       $ 17.19