SEGMENTS INFORMATION - Reconciliation of Segment Revenues and Operating Profit to Consolidated Income From Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 298,293	$ 236,049	$ 846,607	$ 653,875
Segment Reporting Information [Line Items]
Income from operations	33,902	27,776	96,313	74,202
Stock-based compensation expense			(37,520)	(33,350)
Depreciation and amortization	(5,925)	(4,393)	(17,150)	(12,496)
Interest and other income, net	1,067	865	3,416	3,322
Foreign exchange (loss)/gain	(1,728)	32	(5,313)	(6,187)
Income before provision for income taxes	33,241	28,673	94,416	71,337
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	298,354	236,174	847,145	654,437
Segment Reporting Information [Line Items]
Income from operations	55,455	47,721	158,730	130,834
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(61)	(125)	(538)	(562)
Segment Reporting Information [Line Items]
Other income	(61)	(125)	(538)	(562)
Stock-based compensation expense	(13,826)	(11,963)	(37,520)	(33,350)
Non-corporate taxes	(1,588)	(1,120)	(4,105)	(2,858)
Professional fees	(1,470)	(1,572)	(5,656)	(5,423)
Depreciation and amortization	(2,027)	(1,312)	(6,294)	(3,919)
Bank charges	(370)	(340)	(1,097)	(1,021)
One time charges	(7)	0	(314)	0
Other corporate expenses	$ (2,204)	$ (3,513)	$ (6,893)	$ (9,499)