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Stockholder's Equity (Deficit) - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
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    
Treasury stock purchase price   $ 9.80    
Commitments to repurchase capital stock   0    
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans $ 15.3      
Unrecognized expense expected to be recognized period 3 years      
Number of restricted stock shares issued 33,229,000 40,000,000 26,400,000  
Restricted stock fair value market price per share $ 31.60 $ 24.00 $ 13.64  
Shares of vested over period 1 year 3 years    
Restricted shares became fully vested and released as common stock 22,155,000 12,132,000 8,775,000  
Restricted shares outstanding 267,173,000 239,893,000 212,025,000  
Restricted shares forfeited 1,667,000 0    
Montrose Amended and Restated 2013 Stock Option Plan        
Class Of Stock [Line Items]        
Shares outstanding from exercised options 249,200,000 201,600,000    
Stock Options | Montrose Amended & Restated 2017 Stock Incentive Plan | Board of Directors        
Class Of Stock [Line Items]        
Shares of vested over period 4 years      
Stock Options | Montrose Amended & Restated 2017 Stock Incentive Plan | Executive Officers        
Class Of Stock [Line Items]        
Shares of vested over period 3 years      
Private Offering        
Class Of Stock [Line Items]        
Warrants to purchase common stock       116,350
Warrant exercise price per share       $ 17.19