SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, STOCK BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 16,299	$ 16,730	$ 46,988	$ 41,437
Ten-X
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based payment arrangement, expensed and capitalized, amount				700
Cost of revenues
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	2,835	3,007	8,283	8,009
Selling and marketing (excluding customer base amortization)
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	1,620	2,091	4,644	5,886
Software development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	3,316	2,821	9,289	7,635
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 8,528	$ 8,811	$ 24,772	$ 19,907