SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Stock-Based Compensation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 52,255	$ 42,718	$ 39,030
Expense related to stock options settled		1,500
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	9,273	7,688	4,971
Selling and marketing (excluding customer base amortization)
Stock-Based Compensation Expense [Abstract]
Compensation expense	6,809	6,881	7,086
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	8,985	7,454	7,071
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 27,188	$ 20,695	$ 19,902