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
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 324,651	$ 264,482
Segment Reporting Information [Line Items]
Income from operations	30,966	30,331
Stock-based compensation expense	(15,776)	(10,964)
Depreciation and amortization	(6,672)	(5,102)
Interest and other income, net	584	1,211
Foreign exchange loss	(2,955)	(1,290)
Income before provision for income taxes	28,595	30,252
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	324,703	264,814
Segment Reporting Information [Line Items]
Income from operations	60,452	48,647
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(52)	(332)
Segment Reporting Information [Line Items]
Other revenues	(52)	(332)
Stock-based compensation expense	(15,776)	(10,964)
Non-corporate taxes	(3,451)	(1,080)
Professional fees	(2,344)	(1,726)
Depreciation and amortization	(1,975)	(1,691)
Bank charges	(397)	(341)
One-time charges	(568)	0
Other corporate expenses	$ (4,923)	$ (2,182)