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Stockholders' Equity - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2020
Dec. 31, 2019
Dec. 31, 2018
Mar. 16, 2020
May 01, 2018
Class of Stock [Line Items]            
Additional shares issued 266,410          
Shelf Registration [Member]            
Class of Stock [Line Items]            
Additional shares issued   0 0      
Common Class A [Member]            
Class of Stock [Line Items]            
Issuance of shares related to acquisition       163,137    
Common Class A [Member] | Stock Repurchase Program [Member]            
Class of Stock [Line Items]            
Stock repurchase program, authorized amount         $ 250,000,000  
Stock repurchase program expiry date   Sep. 30, 2021        
Stock repurchased during period   $ 0        
Common Class A [Member] | Equity Distribution Agreement [Member]            
Class of Stock [Line Items]            
Common stock, shares sold   842,412        
Common stock, shares available to be sold   $ 336,668,000        
Common Class A [Member] | Maximum [Member]            
Class of Stock [Line Items]            
Aggregate offering price of common stock issuable           $ 400,000,000