SEGMENT INFORMATION (Reconciliation of Segment Revenues and Operating Profit to Consolidated Income From Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 1,160,132	$ 914,128	$ 730,027
Segment Reporting Information [Line Items]
Total segment operating profit:	$ 37,383	$ 33,902	$ 32,080	$ 30,331	$ 31,765	$ 27,776	$ 23,606	$ 22,820	133,696	105,967	86,183
Stock-based compensation expense									(49,244)	(45,833)	(24,620)
Depreciation and amortization	(6,237)	(5,925)	(6,123)	(5,102)	(4,899)	(4,393)	(3,903)	(4,200)	(23,387)	(17,395)	(17,483)
Interest and other income, net	1,432	1,067	1,138	1,211	1,409	865	1,299	1,158	4,848	4,731	4,769
Change in fair value of contingent consideration									0	0	(1,924)
Foreign exchange loss	(6,765)	(1,728)	(2,295)	(1,290)	1,559	32	(465)	(5,754)	(12,078)	(4,628)	(2,075)
Income before provision for income taxes	$ 32,050	$ 33,241	$ 30,923	$ 30,252	$ 34,733	$ 28,673	$ 24,440	$ 18,224	126,466	106,070	86,953
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,160,815	914,966	730,003
Segment Reporting Information [Line Items]
Total segment operating profit:									218,121	186,133	144,619
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									(683)	(838)	24
Segment Reporting Information [Line Items]
Other (revenues)/loss									(683)	(838)	24
Stock-based compensation expense									(49,244)	(45,833)	(24,620)
Non-corporate taxes									(5,909)	(4,274)	(6,882)
Professional fees									(8,265)	(7,104)	(5,312)
Depreciation and amortization									(8,290)	(5,581)	(7,988)
Bank charges									(1,515)	(1,352)	(1,049)
One-time charges									(706)	(747)	(5,983)
Other corporate expenses									$ (9,813)	$ (14,437)	$ (6,626)