SEGMENT INFORMATION (Reconciliation of Segment Revenues and Operating Profit to Consolidated Income Before Providion for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									$ 1,450,448	$ 1,160,132	$ 914,128
Segment Reporting Information [Line Items]
Total segment operating profit:	$ 52,050	$ 49,248	$ 40,682	$ 30,966	$ 37,383	$ 33,902	$ 32,080	$ 30,331	172,946	133,696	105,967
Stock-based compensation expense									(52,407)	(49,244)	(45,833)
Depreciation and amortization	(7,696)	(7,174)	(7,020)	(6,672)	(6,237)	(5,925)	(6,123)	(5,102)	(28,562)	(23,387)	(17,395)
Interest and other income, net	1,799	1,416	802	584	1,432	1,067	1,138	1,211	4,601	4,848	4,731
Foreign exchange loss	(1,772)	(77)	1,562	(2,955)	(6,765)	(1,728)	(2,295)	(1,290)	(3,242)	(12,078)	(4,628)
Income before provision for income taxes	$ 52,077	$ 50,587	$ 43,046	$ 28,595	$ 32,050	$ 33,241	$ 30,923	$ 30,252	174,305	126,466	106,070
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues									1,452,287	1,160,815	914,966
Segment Reporting Information [Line Items]
Total segment operating profit:									275,326	218,121	186,133
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Other income included in segment revenues									(1,839)	(683)	(838)
Segment Reporting Information [Line Items]
Other income included in segment revenues									(1,839)	(683)	(838)
Stock-based compensation expense									(52,407)	(49,244)	(45,833)
Non-corporate taxes									(9,659)	(5,909)	(4,274)
Professional fees									(8,032)	(8,265)	(7,104)
Depreciation and amortization									(7,632)	(8,290)	(5,581)
Bank charges									(1,969)	(1,515)	(1,352)
One-time charges									(1,741)	(706)	(747)
Other corporate expenses									$ (19,101)	$ (9,813)	$ (14,437)