OPERATING SEGMENTS (Reconciliation of Segment Revenues and Operating Profit to Consolidated Income From Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 914,128	$ 730,027	$ 555,117
Segment Reporting Information [Line Items]
Total segment operating profit:	$ 31,765	$ 27,776	$ 23,606	$ 22,820	$ 24,015	$ 21,835	$ 18,476	$ 21,857	105,967	86,183	76,493
Stock-based compensation expense									(45,833)	(24,620)	(13,150)
Depreciation and amortization	(4,899)	(4,393)	(3,903)	(4,200)	(2,833)	(5,510)	(5,451)	(3,689)	(17,395)	(17,483)	(15,120)
Provision for bad debts									(1,704)	(1,325)	(619)
Interest and other income, net	1,409	865	1,299	1,158	1,368	1,261	1,164	976	4,731	4,769	3,077
Change in fair value of contingent consideration	0	0	0	0	(1,924)	0	0	0	0	(1,924)	0
Foreign exchange loss	1,559	32	(465)	(5,754)	1,123	(718)	(1,239)	(1,241)	(4,628)	(2,075)	(2,800)
Income before provision for income taxes	$ 34,733	$ 28,673	$ 24,440	$ 18,224	$ 24,582	$ 22,378	$ 18,401	$ 21,592	106,070	86,953	76,770
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									914,966	730,003	555,043
Segment Reporting Information [Line Items]
Total segment operating profit:									186,133	144,619	109,308
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									(838)	24	74
Segment Reporting Information [Line Items]
Other (revenues)/loss									(838)	24	74
Stock-based compensation expense									(45,833)	(24,620)	(13,150)
Non-corporate taxes									(4,274)	(6,882)	(3,201)
Professional fees									(7,104)	(5,312)	(3,651)
Depreciation and amortization									(5,581)	(7,988)	(2,829)
Bank charges									(1,352)	(1,049)	(1,194)
One-time charges									(747)	(5,983)	0
Provision for bad debts									0	0	(36)
Other corporate expenses									$ (14,437)	$ (6,626)	$ (8,828)