SEGMENTS INFORMATION - Reconciliation of Segment Revenues to Consolidated Revenues and Segment Operating Profit to Consolidated Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 445,647	$ 348,977	$ 869,795	$ 673,628
Segment Reporting Information [Line Items]
Income from operations	54,237	40,682	102,934	71,648
Stock-based compensation expense			(31,406)	(28,703)
Depreciation and amortization expense	(8,962)	(7,020)	(17,138)	(13,692)
Interest and other income, net	1,052	802	501	1,386
Foreign exchange gain/(loss)	1,830	1,562	1,583	(1,393)
Income before provision for income taxes	57,119	43,046	105,018	71,641
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	445,978	349,021	870,303	673,724
Segment Reporting Information [Line Items]
Income from operations	80,110	66,175	158,307	126,627
Other Income Included in Segment Revenues
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(331)	(44)	(508)	(96)
Segment Reporting Information [Line Items]
Other income included in segment revenues	(331)	(44)	(508)	(96)
Stock-based compensation expense	(14,810)	(12,927)	(31,406)	(28,703)
Non-corporate taxes	(2,418)	(1,795)	(4,978)	(5,246)
Professional fees	(1,400)	(2,476)	(3,316)	(4,820)
Depreciation and amortization expense	(1,974)	(1,905)	(3,743)	(3,880)
Bank charges	(579)	(446)	(1,168)	(843)
One-time charges and other acquisition-related expenses	(1,241)	(330)	(1,861)	(898)
Other operating expenses	$ (3,120)	$ (5,570)	$ (8,393)	$ (10,493)