SEGMENT INFORMATION - Revenue Disaggregated by Service (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information
Revenue from contracts with customers	$ 294,822	$ 180,711	$ 550,133	$ 331,456
North America
Segment Reporting Information
Revenue from contracts with customers	277,505	165,491	513,531	303,563
Europe
Segment Reporting Information
Revenue from contracts with customers	17,317	15,220	36,602	27,893
Marketplace revenue | North America
Segment Reporting Information
Revenue from contracts with customers	204,735	155,755	370,343	285,399
Consumer connection revenue | North America
Segment Reporting Information
Revenue from contracts with customers	187,172	141,163	336,232	257,163
Membership subscription revenue | North America
Segment Reporting Information
Revenue from contracts with customers	16,565	13,704	32,192	26,456
Other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	998	888	1,919	1,780
Advertising & Other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	72,770	9,736	143,188	18,164
Advertising & Other revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	304	178	551	380
Consumer connection revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	12,496	11,170	26,863	19,635
Membership subscription revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	$ 4,517	$ 3,872	$ 9,188	$ 7,878