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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 348,977	$ 283,832	$ 673,628	$ 548,314
Segment Reporting Information [Line Items]
Income from operations	40,682	32,080	71,648	62,411
Stock-based compensation expense			(28,703)	(23,694)
Depreciation and amortization	(7,020)	(6,123)	(13,692)	(11,225)
Interest and other income, net	802	1,138	1,386	2,349
Foreign exchange gain (loss)	1,562	(2,295)	(1,393)	(3,585)
Income before provision for income taxes	43,046	30,923	71,641	61,175
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	349,021	283,977	673,724	548,791
Segment Reporting Information [Line Items]
Income from operations	66,175	54,628	126,627	103,275
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(44)	(145)	(96)	(477)
Segment Reporting Information [Line Items]
Other revenues	(44)	(145)	(96)	(477)
Stock-based compensation expense	(12,927)	(12,730)	(28,703)	(23,694)
Taxes other than income taxes	(1,795)	(1,437)	(5,246)	(2,517)
Professional fees	(2,476)	(2,460)	(4,820)	(4,186)
Depreciation and amortization	(1,905)	(2,576)	(3,880)	(4,267)
Bank charges	(446)	(386)	(843)	(727)
Other acquisition-related expenses	(330)	(307)	(898)	(307)
Other corporate expenses	$ (5,570)	$ (2,507)	$ (10,493)	$ (4,689)