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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
Jun. 28, 2019
Dec. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Feb. 28, 2018
Feb. 08, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Monthly Average Closing Price, Common Stock, Threshold $ 33.52 $ 10.83            
Stock Issued During Period, Shares, Employee Stock Purchase Plans         93,584      
Number of shares authorized (in shares)               1,875,000
Number of shares authorized, annual increase   5.00%            
Total stock-based compensation expense     $ 7,486 $ 5,723 $ 12,266 $ 16,968    
Delivery costs                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Total stock-based compensation expense     176 203 539 471    
Sales and marketing expense                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Total stock-based compensation expense     1,432 1,939 3,091 5,550    
Research and development expense                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Total stock-based compensation expense     638 915 1,204 3,141    
General and administration expense                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Total stock-based compensation expense     $ 5,240 $ 2,666 $ 7,432 $ 7,806    
ESPP                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
ESPP, number of shares authorized, annual increase (in shares)     500,000   500,000      
Number of shares authorized, annual increase   1.00%            
2018 Employee Stock Purchase Plan [Member] | ESPP                
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]                
Number of shares authorized (in shares)             375,000