GOODWILL AND OTHER INTANGIBLE ASSETS - Amortization Expense of Acquired Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets [Line Items]
Total amortization expense	$ 11,443	$ 12,338	$ 13,408
Cost of revenue
Acquired Finite-Lived Intangible Assets [Line Items]
Total amortization expense	5,027	5,103	5,986
Selling and marketing
Acquired Finite-Lived Intangible Assets [Line Items]
Total amortization expense	6,416	7,235	7,145
General and administrative
Acquired Finite-Lived Intangible Assets [Line Items]
Total amortization expense	$ 0	$ 0	$ 277