STOCK-BASED COMPENSATION - Allocation of Stock-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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			$ 69.98
Number of shares authorized (in shares)						1,345,631		1,875,000
Total stock-based compensation expense	$ 9,108	$ 3,072	$ 13,233	$ 4,780
Delivery costs
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	357	199	532	363
Sales and marketing expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	2,567	952	3,836	1,659
Research and development expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	1,401	363	2,004	566
General and administration expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 4,783	$ 1,558	$ 6,861	$ 2,192
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%