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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 377,523	$ 298,293	$ 1,051,151	$ 846,607
Segment Reporting Information [Line Items]
Income from operations	49,248	33,902	120,896	96,313
Stock-based compensation expense			(39,920)	(37,520)
Depreciation and amortization	(7,174)	(5,925)	(20,866)	(17,150)
Interest and other income, net	1,416	1,067	2,802	3,416
Foreign exchange loss	(77)	(1,728)	(1,470)	(5,313)
Income before provision for income taxes	50,587	33,241	122,228	94,416
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	378,290	298,354	1,052,014	847,145
Segment Reporting Information [Line Items]
Income from operations	73,198	55,455	199,825	158,730
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(767)	(61)	(863)	(538)
Segment Reporting Information [Line Items]
Other revenues	(767)	(61)	(863)	(538)
Stock-based compensation expense	(11,217)	(13,826)	(39,920)	(37,520)
Taxes other than income taxes	(1,966)	(1,588)	(7,212)	(4,105)
Professional fees	(1,050)	(1,470)	(5,870)	(5,656)
Depreciation and amortization	(1,883)	(2,027)	(5,763)	(6,294)
Bank charges	(574)	(370)	(1,417)	(1,097)
One-time charges and other acquisition-related expenses	(289)	(7)	(1,187)	(314)
Other corporate expenses	$ (6,204)	$ (2,204)	$ (16,697)	$ (6,893)