SEGMENT INFORMATION - Revenue Disaggregated by Service (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue	$ 278,992	$ 303,116	$ 294,822	$ 255,311	$ 223,213	$ 181,717	$ 180,711	$ 150,745	$ 1,132,241	$ 736,386	$ 498,890
Felix
Segment Reporting Information [Line Items]
Revenue									36,900
North America
Segment Reporting Information [Line Items]
Revenue									1,062,171	678,897	461,847
Europe
Segment Reporting Information [Line Items]
Revenue									70,070	57,489	37,043
Consumer connection revenue | North America
Segment Reporting Information [Line Items]
Revenue									704,341	521,481	382,466
Consumer connection revenue | Europe
Segment Reporting Information [Line Items]
Revenue									50,913	40,009	28,124
Membership subscription revenue | North America
Segment Reporting Information [Line Items]
Revenue									66,214	56,135	43,573
Membership subscription revenue | Europe
Segment Reporting Information [Line Items]
Revenue									17,362	16,596	7,936
Other revenue | North America
Segment Reporting Information [Line Items]
Revenue									3,940	3,798	2,827
Marketplace revenue | North America
Segment Reporting Information [Line Items]
Revenue									774,495	581,414	428,866
Advertising and other revenue | North America
Segment Reporting Information [Line Items]
Revenue									287,676	97,483	32,981
Advertising and other revenue | Europe
Segment Reporting Information [Line Items]
Revenue									$ 1,795	$ 884	$ 983