SEGMENT INFORMATION (Reconciliation of Segment Revenues and Operating Profit to Consolidated Income Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues	$ 504,931	$ 468,186	$ 445,647	$ 424,148	$ 399,297	$ 377,523	$ 348,977	$ 324,651	$ 1,842,912	$ 1,450,448	$ 1,160,132
Segment Reporting Information [Line Items]
Total segment operating profit:	78,270	64,560	54,237	48,697	52,050	49,248	40,682	30,966	245,764	172,946	133,696
Stock-based compensation expense									(59,188)	(52,407)	(49,244)
Depreciation and amortization	(10,183)	(9,319)	(8,962)	(8,176)	(7,696)	(7,174)	(7,020)	(6,672)	(36,640)	(28,562)	(23,387)
Interest and other income, net	1,080	1,941	1,052	(551)	1,799	1,416	802	584	3,522	4,601	4,848
Foreign exchange gain/(loss)	(582)	(514)	1,830	(247)	(1,772)	(77)	1,562	(2,955)	487	(3,242)	(12,078)
Income before provision for income taxes	$ 78,768	$ 65,987	$ 57,119	$ 47,899	$ 52,077	$ 50,587	$ 43,046	$ 28,595	249,773	174,305	126,466
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment Revenues									1,844,109	1,452,287	1,160,815
Segment Reporting Information [Line Items]
Total segment operating profit:									349,099	275,326	218,121
Unallocated Amounts
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment Revenues									(1,197)	(1,839)	(683)
Segment Reporting Information [Line Items]
Other income included in segment revenues									(1,197)	(1,839)	(683)
Stock-based compensation expense									(59,188)	(52,407)	(49,244)
Non-corporate taxes									(9,856)	(9,659)	(5,909)
Professional fees									(6,188)	(8,032)	(8,265)
Depreciation and amortization									(8,057)	(7,632)	(8,290)
Bank charges									(2,358)	(1,969)	(1,515)
One-time charges and other acquisition-related expenses									(2,055)	(1,741)	(706)
Other operating expenses									$ (14,436)	$ (19,101)	$ (9,813)