INFORMATION ABOUT REVENUE AND GEOGRAPHIC AREAS (Net Revenue) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	$ 846,813	$ 713,069	$ 549,711
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	832,732	698,244	537,567
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	14,081	14,825	12,144
Advertising
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	771,644	645,241	471,416
Transactions
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	60,251	62,495	43,854
Other services
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	14,918	5,333	3,429
Brand advertising
Revenues from External Customers and Long-Lived Assets [Line Items]
Net revenue	$ 0	$ 0	$ 31,012