SEGMENTS INFORMATION (Reconciliation of Segment Revenues to Consolidated Revenues and Segment Operating Profit to Consolidated Income Before Provision for/(Benefit from) Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 588,103	$ 468,186	$ 1,661,023	$ 1,337,981
Segment Reporting Information [Line Items]
Income from operations	80,565	64,560	218,105	167,494
Stock-based compensation expense			(53,024)	(46,736)
Depreciation and amortization expense	(11,127)	(9,319)	(32,355)	(26,457)
Interest and other income, net	2,509	1,941	6,775	2,442
Foreign exchange (loss)/gain	(3,105)	(514)	(10,151)	1,069
Income before provision for income taxes	79,969	65,987	214,729	171,005
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	588,274	468,528	1,661,458	1,338,831
Segment Reporting Information [Line Items]
Income from operations	112,384	90,177	311,893	248,484
Unallocated amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment revenues	(171)	(342)	(435)	(850)
Segment Reporting Information [Line Items]
Other income included in segment revenues	(171)	(342)	(435)	(850)
Stock-based compensation expense	(15,471)	(15,330)	(53,024)	(46,736)
Non-corporate taxes	(3,960)	(2,063)	(8,126)	(7,041)
Professional fees	(1,502)	(1,420)	(3,826)	(4,736)
Depreciation and amortization expense	(2,585)	(2,011)	(7,328)	(5,754)
Bank charges	(645)	(782)	(1,937)	(1,950)
One-time charges and other acquisition-related expenses	(1,142)	(155)	(2,503)	(2,016)
Other operating expenses	$ (6,343)	$ (3,514)	$ (16,609)	$ (11,907)