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STOCK-BASED COMPENSATION - Allocation of Stock-based Compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Jan. 01, 2020
Dec. 31, 2019
Dec. 31, 2018
Feb. 08, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Monthly Average Closing Price, Common Stock, Threshold     $ 70.57          
Number of shares authorized (in shares)           1,345,631   1,875,000
Total stock-based compensation expense $ 11,578 $ 7,486 $ 24,811 $ 12,266        
Delivery costs                
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
Total stock-based compensation expense 365 176 897 539        
Sales and marketing expense                
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
Total stock-based compensation expense 3,791 1,432 7,627 3,091        
Research and development expense                
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
Total stock-based compensation expense 1,510 638 3,514 1,204        
General and administration expense                
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
Total stock-based compensation expense $ 5,912 $ 5,240 $ 12,773 $ 7,432        
ESPP                
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]                
ESPP, number of shares authorized, annual increase (in shares) 500,000   500,000   265,470,000 267,823,000    
Number of shares authorized, annual increase             1.00%