SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Property, Equipment and Software) (Details)	12 Months Ended
	Dec. 31, 2017
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Capitalized website and internal-use software development costs
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years