SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI, Current Period, Tax			$ 200
Revenue from Contract with Customers, Revenue constrained and to be recognized in future periods		$ 14,600	13,700
Hosting Arrangement, Service Contract, Implementation Cost, Capitalized, Amount Capitalized		7,400
Prepaid expenses and other current assets		82,073	88,115
Other non-current assets		28,605	20,989
Accrued merchant and supplier payables		540,940	651,781
Accrued expenses and other current liabilities		260,192	267,034
Other non-current liabilities		44,987	100,688
Accumulated deficit		1,032,876	1,010,499
Amortization		20,400	25,200
Provision (benefit) for income taxes		761	(957)	$ 7,544
Accounting Standards Update 2016-16 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle, net of tax			(88,945)	3,234
Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification				$ 7,000
Prepaid Expenses and Other Current Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Costs		2,501	2,923
Other Noncurrent Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Costs		$ 10,133	$ 11,285
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		10 years
Maximum | Computer hardware
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Maximum | Furniture and fixtures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life		10 years
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		1 year
Minimum | Computer hardware
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Minimum | Furniture and fixtures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Internally-developed software
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life		2 years
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09
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
Provision (benefit) for income taxes	$ 6,700