ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Loyalty Deferred Revenue (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Loyalty program
Loyalty deferred revenue, beginning balance	$ 6,406	$ 7,033
Loyalty points forfeited	(2,131)	(5,895)
Loyalty points used	(6,754)	(14,316)
Loyalty points vested	6,345	17,836
Loyalty points unvested	1,091	1,748
Loyalty deferred revenue, ending balance	$ 4,957	$ 6,406