OPERATING SEGMENTS - Reconciliation of Segment Revenues and Operating Profit to Consolidated Income From Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 264,482	$ 200,045
Segment Reporting Information [Line Items]
Income from operations	30,331	22,820
Stock-based compensation expense	(10,964)	(9,134)
Depreciation and amortization	(5,102)	(4,200)
Interest and other income, net	1,211	1,158
Foreign exchange loss	(1,290)	(5,754)
Income before provision for income taxes	30,252	18,224
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	264,814	200,194
Segment Reporting Information [Line Items]
Income from operations	48,647	38,680
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	(332)	(149)
Segment Reporting Information [Line Items]
Unallocated other revenues/ (discounts)	(332)	(149)
Stock-based compensation expense	(10,964)	(9,134)
Non-corporate taxes	(1,080)	(825)
Professional fees	(1,726)	(1,822)
Depreciation and amortization	(1,691)	(1,329)
Bank charges	(341)	(367)
Other corporate expenses	$ (2,182)	$ (2,234)