SEGMENT INFORMATION - Revenue Disaggregated by Service (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Revenue from contracts with customers	$ 303,116	$ 181,717	$ 853,249	$ 513,173
North America
Segment Reporting Information
Revenue from contracts with customers	286,594	167,104	800,125	470,667
Europe
Segment Reporting Information
Revenue from contracts with customers	16,522	14,613	53,124	42,506
Marketplace revenue | North America
Segment Reporting Information
Revenue from contracts with customers	213,049	156,601	583,392	442,000
Consumer connection revenue | North America
Segment Reporting Information
Revenue from contracts with customers	195,065	141,055	531,297	398,218
Membership subscription revenue | North America
Segment Reporting Information
Revenue from contracts with customers	17,034	14,486	49,226	40,942
Other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	950	1,060	2,869	2,840
Advertising & Other revenue | North America
Segment Reporting Information
Revenue from contracts with customers	73,545	10,503	216,733	28,667
Advertising & Other revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	283	292	834	672
Consumer connection revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	12,022	10,001	38,885	29,636
Membership subscription revenue | Europe
Segment Reporting Information
Revenue from contracts with customers	$ 4,217	$ 4,320	$ 13,405	$ 12,198