ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued compensation and other related costs	$ 38,364	$ 13,012
Accounts payable accruals	27,198	15,692
Accrued marketing expenses	21,057	13,063
Allowance for returns	17,916	11,107	$ 15,261
Sales and other taxes payable	15,623	10,105
Accrued freight	12,773	5,954
Other accrued expenses	10,157	9,714
Accrued loss contingencies	2,580	9,550
Total accrued liabilities	$ 145,668	$ 88,197