ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Loyalty Deferred Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Loyalty program
Loyalty deferred revenue, beginning balance	$ 4,231	$ 6,406
Loyalty points forfeited or expired	(1,585)	(4,332)
Loyalty points used	(2,329)	(11,398)
Loyalty points vested	1,906	12,469
Loyalty points unvested	1,185	1,086
Loyalty deferred revenue, ending balance	$ 3,408	$ 4,231