ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) performance_obligation region	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of regions in which company sells products | region	3
Credit card receivables	$ 2,800	$ 1,600
Cash and cash equivalents held in foreign bank accounts	15,300	19,900
ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Line Items]
Restricted cash	9,500	8,700
Accounts receivable, net of allowance	143	106
Allowance for doubtful accounts	800	800	$ 600
Other assets	8,595	3,894
Deposits for building leases	1,900	2,000	1,900
Fair trade commission deposits	1,600	1,700	1,700
Indefinite lived intangible assets	200	200
Notes payable	1,700	1,600
Notes payable, current portion	975	702
Other long-term liabilities	5,575	2,302
Uncertain income tax position	200	200	200
Accrued lease restoration costs	400	400	400
Defined benefit plan obligation	300	400	$ 400
Lease incentive payable, noncurrent	1,300
Deferred commissions	2,420	2,449
Amortization of deferred commissions	1,200
Deferred revenue	5,393	5,274
Deferred revenue, revenue recognized during the period	4,600
Loyalty deferred revenue, ending balance	$ 3,408
Percentage of sale returns	1.50%
Revenue, number of performance obligations, product sales | performance_obligation	2
Revenue, performance obligations, term of services, online business tools	3 months
Revenue, number of performance obligations, online business tools | performance_obligation	3
Operating Lease, Right-of-Use Asset	$ 4,800
Operating Lease, Liability, Noncurrent	3,300
South Korea
ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Line Items]
Net assets	$ 15,500	$ 14,400
Software Tools
ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	3 months
Associate Fees
ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	12 months