REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 521,333	$ 424,148
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	451	605
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	520,882	423,543
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	464,210	383,878
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	55,273	38,617
Licensing
Disaggregation of Revenue [Line Items]
Revenues	1,240	1,291
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	610	362
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	113,261	103,822
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	104,326	91,814
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	99,931	81,622
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	95,032	76,207
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	54,774	32,290
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	54,009	38,393
North America
Disaggregation of Revenue [Line Items]
Revenues	316,652	239,446
Europe
Disaggregation of Revenue [Line Items]
Revenues	173,508	153,085
CIS
Disaggregation of Revenue [Line Items]
Revenues	18,174	21,780
APAC
Disaggregation of Revenue [Line Items]
Revenues	12,999	9,837
North America
Disaggregation of Revenue [Line Items]
Segment revenues	310,689	231,196
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	404	344
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	310,285	230,852
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	284,725	214,316
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	24,740	15,992
North America | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	836	662
North America | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	388	226
North America | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	38,394	21,956
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	47,000	40,687
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	79,121	60,564
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	62,361	57,337
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	50,156	27,467
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	33,657	23,185
North America | North America
Disaggregation of Revenue [Line Items]
Segment revenues	303,745	226,070
North America | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	4,747	2,740
North America | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	1,739	2,003
North America | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	458	383
Europe
Disaggregation of Revenue [Line Items]
Segment revenues	194,223	173,343
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	205	428
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	194,018	172,915
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	168,213	159,466
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	25,246	12,953
Europe | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	398	619
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	366	305
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	61,988	65,818
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	55,202	49,454
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	20,370	20,294
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	32,536	18,870
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	4,556	4,823
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	19,571	14,084
Europe | North America
Disaggregation of Revenue [Line Items]
Segment revenues	12,891	13,361
Europe | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	168,767	150,480
Europe | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	13	63
Europe | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	12,552	9,439
Russia
Disaggregation of Revenue [Line Items]
Segment revenues	16,580	19,786
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	1	10
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	16,579	19,776
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	11,272	10,096
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	5,287	9,672
Russia | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	6	10
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	15	8
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	13,032	16,225
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	2,124	1,673
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	440	764
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	140	0
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	62	0
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	782	1,124
Russia | North America
Disaggregation of Revenue [Line Items]
Segment revenues	16	15
Russia | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	141	42
Russia | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	16,423	19,714
Russia | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	0	15
Total Segments
Disaggregation of Revenue [Line Items]
Segment revenues	521,492	424,325
Total Segments | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	610	782
Total Segments | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	520,882	423,543
Total Segments | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	464,210	383,878
Total Segments | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	55,273	38,617
Total Segments | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	1,240	1,291
Total Segments | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	769	539
Total Segments | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	113,414	103,999
Total Segments | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	104,326	91,814
Total Segments | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	99,931	81,622
Total Segments | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	95,037	76,207
Total Segments | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	54,774	32,290
Total Segments | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	54,010	38,393
Total Segments | North America
Disaggregation of Revenue [Line Items]
Segment revenues	316,652	239,446
Total Segments | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	173,655	153,262
Total Segments | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	18,175	21,780
Total Segments | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	13,010	9,837
Other Income Included in Segment Revenues
Disaggregation of Revenue [Line Items]
Segment revenues	(159)	(177)
Other income included in segment revenues	(159)	(177)
Other Income Included in Segment Revenues | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(159)	(177)
Other Income Included in Segment Revenues | Transferred over time
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Time-and-material
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Fixed-price
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Licensing
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(159)	(177)
Other Income Included in Segment Revenues | Financial Services
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(153)	(177)
Other Income Included in Segment Revenues | Travel & Consumer
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Business Information & Media
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(5)	0
Other Income Included in Segment Revenues | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Emerging Verticals
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(1)	0
Other Income Included in Segment Revenues | North America
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0
Other Income Included in Segment Revenues | Europe
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(147)	(177)
Other Income Included in Segment Revenues | CIS
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(1)	0
Other Income Included in Segment Revenues | APAC
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	$ (11)	$ 0