STOCK-BASED COMPENSATION - Expense Included in Consolidated Statements of Operations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax	[1]	$ 63,862	$ 53,313	$ 40,821
Acquisition-related	[2]	0	0	2,903
Income tax benefit		(13,383)	(12,113)	(12,198)
Cost of revenues
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax		16,862	14,573	11,459
Selling and marketing
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax		23,237	15,720	12,464
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax		15,274	13,618	10,043
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax		8,489	9,402	6,513
Acquisition-related
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Acquisition-related		$ 0	$ 0	$ 342

[1] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.

[2] On January 1, 2018, the Company adopted the ASC 606 revenue recognition standard and adjusted prior periods to conform. See "Note 2. Significant Accounting Policies" for additional information.