REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 632,775	$ 588,103	$ 551,587	$ 521,333	$ 504,931	$ 468,186	$ 445,647	$ 424,148	$ 2,293,798	$ 1,842,912	$ 1,450,448
Transferred Over Time
Disaggregation of Revenue [Line Items]
Revenues									2,291,245	1,841,242
Transferred at a Point of Time
Disaggregation of Revenue [Line Items]
Revenues									2,553	1,670
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues									1,990,653	1,652,897
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									294,650	184,251
Licensing
Disaggregation of Revenue [Line Items]
Revenues									5,081	4,097
Other revenues
Disaggregation of Revenue [Line Items]
Revenues									3,414	1,667
Financial Services
Disaggregation of Revenue [Line Items]
Revenues									500,872	423,977
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues									439,358	393,643
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues									433,398	350,815
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues									420,923	324,033
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues									248,452	171,703
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues									250,795	178,741
North America
Disaggregation of Revenue [Line Items]
Revenues									1,390,015	1,099,167
Europe
Disaggregation of Revenue [Line Items]
Revenues									746,866	612,472
CIS
Disaggregation of Revenue [Line Items]
Revenues									100,471	81,703
APAC
Disaggregation of Revenue [Line Items]
Revenues									56,446	49,570
North America Segment
Disaggregation of Revenue [Line Items]
Revenues									1,380,944	1,076,979	796,040
North America Segment | Transferred Over Time
Disaggregation of Revenue [Line Items]
Revenues									1,379,256	1,076,084
North America Segment | Transferred at a Point of Time
Disaggregation of Revenue [Line Items]
Revenues									1,688	895
North America Segment | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues									1,247,979	983,436
North America Segment | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									127,926	89,831
North America Segment | Licensing
Disaggregation of Revenue [Line Items]
Revenues									3,626	2,748
North America Segment | Other revenues
Disaggregation of Revenue [Line Items]
Revenues									1,413	964
North America Segment | Financial Services
Disaggregation of Revenue [Line Items]
Revenues									184,469	112,444
North America Segment | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues									198,264	177,910
North America Segment | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues									354,023	269,067
North America Segment | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues									262,448	251,081
North America Segment | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues									224,925	151,418
North America Segment | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues									156,815	115,059
North America Segment | North America
Disaggregation of Revenue [Line Items]
Revenues									1,344,040	1,046,232
North America Segment | Europe
Disaggregation of Revenue [Line Items]
Revenues									27,042	16,679
North America Segment | CIS
Disaggregation of Revenue [Line Items]
Revenues									8,583	8,437
North America Segment | APAC
Disaggregation of Revenue [Line Items]
Revenues									1,279	5,631
Europe Segment
Disaggregation of Revenue [Line Items]
Revenues									820,717	692,785	591,450
Europe Segment | Transferred Over Time
Disaggregation of Revenue [Line Items]
Revenues									819,913	692,023
Europe Segment | Transferred at a Point of Time
Disaggregation of Revenue [Line Items]
Revenues									804	762
Europe Segment | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues									688,605	628,707
Europe Segment | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									128,977	62,078
Europe Segment | Licensing
Disaggregation of Revenue [Line Items]
Revenues									1,230	1,332
Europe Segment | Other revenues
Disaggregation of Revenue [Line Items]
Revenues									1,905	668
Europe Segment | Financial Services
Disaggregation of Revenue [Line Items]
Revenues									244,284	252,196
Europe Segment | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues									229,523	208,266
Europe Segment | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues									77,377	79,121
Europe Segment | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues									157,844	72,898
Europe Segment | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues									23,444	20,272
Europe Segment | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues									88,245	60,032
Europe Segment | North America
Disaggregation of Revenue [Line Items]
Revenues									45,859	52,860
Europe Segment | Europe
Disaggregation of Revenue [Line Items]
Revenues									719,548	595,741
Europe Segment | CIS
Disaggregation of Revenue [Line Items]
Revenues									143	336
Europe Segment | APAC
Disaggregation of Revenue [Line Items]
Revenues									55,167	43,848
Russia Segment
Disaggregation of Revenue [Line Items]
Revenues									92,137	73,148	$ 62,958
Russia Segment | Transferred Over Time
Disaggregation of Revenue [Line Items]
Revenues									92,076	73,135
Russia Segment | Transferred at a Point of Time
Disaggregation of Revenue [Line Items]
Revenues									61	13
Russia Segment | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues									54,069	40,754
Russia Segment | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									37,747	32,342
Russia Segment | Licensing
Disaggregation of Revenue [Line Items]
Revenues									225	17
Russia Segment | Other revenues
Disaggregation of Revenue [Line Items]
Revenues									96	35
Russia Segment | Financial Services
Disaggregation of Revenue [Line Items]
Revenues									72,119	59,337
Russia Segment | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues									11,571	7,467
Russia Segment | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues									1,998	2,627
Russia Segment | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues									631	54
Russia Segment | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues									83	13
Russia Segment | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues									5,735	3,650
Russia Segment | North America
Disaggregation of Revenue [Line Items]
Revenues									116	75
Russia Segment | Europe
Disaggregation of Revenue [Line Items]
Revenues									276	52
Russia Segment | CIS
Disaggregation of Revenue [Line Items]
Revenues									91,745	72,930
Russia Segment | APAC
Disaggregation of Revenue [Line Items]
Revenues									$ 0	$ 91