SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Balance Sheet Accounts Impacted by Cumulative Effect of Adoption (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid expenses and other current assets	$ 40,441	$ 82,073
Other non-current assets	34,327	28,605
Accrued merchant and supplier payables	410,963	540,940
Accrued expenses and other current liabilities	(294,999)	(260,192)
Accumulated deficit	$ 1,320,886	$ 1,032,876
Difference between Revenue Guidance in Effect before and after Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid expenses and other current assets			$ (4,007)
Other non-current assets			(10,223)
Accrued merchant and supplier payables			64,970
Accrued expenses and other current liabilities			(13,188)
Other non-current liabilities			3,443
Accumulated deficit			$ (88,945)