GOODWILL AND LONG-LIVED ASSETS - Long-Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 22,351	$ 0
Warehouse equipment
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			0
Furniture and fixtures
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			413
Leasehold improvements
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			7,558
Office equipment
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			198
Purchased software
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			14
Computer hardware
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			2,842
Right-of-use assets - Finance leases, net
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			1,318
Capitalized software
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			304
Internally-developed software
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			2,988
Total Property, equipment and software, net
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			15,635
Right-of-use assets - operating leases, net
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			19,645
Intangible assets, net
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			103
Other non-current assets
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			871
Long-lived assets
Schedule of Impaired Long-Lived Assets [Line Items]
Impairment of long-lived assets			$ 36,254