STOCK-BASED COMPENSATION - Costs Related To Stock Compensation Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 12,927	$ 12,730	$ 28,703	$ 23,694
Cost of revenues
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	4,189	4,438	9,539	8,082
Selling, general and administrative expenses | Business acquisitions
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	3,277	2,970	7,851	5,980
Selling, general and administrative expenses | Organic growth
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 5,461	$ 5,322	$ 11,313	$ 9,632