SEGMENT INFORMATION - Segmented Revenue Disaggregated by Service (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 357,358	$ 303,116	$ 1,004,697	$ 853,249
North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	339,144	286,594	945,538	800,125
Europe
Segment Reporting Information [Line Items]
Revenue from contracts with customers	18,214	16,522	59,159	53,124
Marketplace revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	270,516	213,049	749,969	583,392
Consumer connection revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	252,552	195,065	695,370	531,297
Membership subscription revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,237	17,034	49,239	49,226
Other revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	1,727	950	5,360	2,869
Advertising and other revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers	68,628	73,545	195,569	216,733
Advertising and other revenue | Europe
Segment Reporting Information [Line Items]
Revenue from contracts with customers	624	283	1,859	834
Consumer connection revenue | Europe
Segment Reporting Information [Line Items]
Revenue from contracts with customers	14,125	12,022	46,480	38,885
Membership subscription revenue | Europe
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 3,465	4,217	$ 10,820	13,405
Felix | Advertising and other revenue | North America
Segment Reporting Information [Line Items]
Revenue from contracts with customers		$ 10,200		$ 28,700