ACCOUNTING POLICIES - Schedule of Costs of Goods Sold (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue recognition
Product delivery period from date of shipment, minimum									1 day
Product delivery period from date of shipment, maximum									8 days
Club O loyalty program
Loyalty program expiration period									90 days
Co-branded credit card program
Maximum revenue from co-branded credit card program as a percentage of total net revenues										1.00%	1.00%
Sales returns allowance
Sales return period for which full refund will be granted									30 days
Sales return period for which reduced refund will be granted									30 days
Sales return received at returns processing facility for which reduced refund will be granted, period									45 days
Cost of goods sold
Total net revenue	$ 452,548	$ 440,580	$ 483,133	$ 445,331	$ 456,290	$ 424,007	$ 432,024	$ 432,435	$ 1,821,592	$ 1,744,756	$ 1,799,963
Total revenue, net (as a percent)									100.00%	100.00%	100.00%
Cost of goods sold
Product costs and other cost of goods sold									$ 1,390,750	$ 1,328,749	$ 1,391,736
Fulfillment and related costs									76,934	75,456	76,878
Total cost of goods sold	370,968	353,864	391,390	351,462	370,492	340,332	347,853	345,528	$ 1,467,684	$ 1,404,205	$ 1,468,614
Product costs and other cost of goods sold (as a percent)									76.00%	76.00%	77.00%
Fulfillment and related costs (as a percent)									4.00%	4.00%	4.00%
Total cost of goods sold (as a percent)									81.00%	80.00%	82.00%
Gross profit	$ 81,580	$ 86,716	$ 91,743	$ 93,869	$ 85,798	$ 83,675	$ 84,171	$ 86,907	$ 353,908	$ 340,551	$ 331,349
Gross profit (as a percent)									19.00%	20.00%	18.00%