SEGMENT INFORMATION - Revenue Disaggregated by Service (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Revenue from contracts with customers	$ 303,443	$ 255,311
North America
Segment Reporting Information
Revenue from contracts with customers	281,994	236,026
Europe
Segment Reporting Information
Revenue from contracts with customers	21,449	19,285
Marketplace revenue | North America
Segment Reporting Information
Revenue from contracts with customers	219,925	165,608
Consumer connection revenue | North America
Segment Reporting Information
Revenue from contracts with customers	201,582	149,060
Membership subscription revenue | North America
Segment Reporting Information
Revenue from contracts with customers	16,517	15,627
Other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	1,826	921
Advertising and other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	62,069	70,418
Advertising and other revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	584	247
Consumer connection revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	17,123	14,367
Membership subscription revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	3,742	$ 4,671
Felix | Advertising and other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	$ 8,500