REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 468,186	$ 377,523	$ 1,337,981	$ 1,051,151
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	141		1,343
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	468,045		1,336,638
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	421,787		1,205,814
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	45,001		127,789
Licensing
Disaggregation of Revenue [Line Items]
Revenues	972		3,229
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	426		1,149
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	104,798		311,608
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	101,093		293,923
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	88,176		254,815
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	81,802		235,658
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	44,609		115,424
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	47,708		126,553
North America
Disaggregation of Revenue [Line Items]
Revenues	284,077		787,974
Europe
Disaggregation of Revenue [Line Items]
Revenues	152,235		455,124
CIS
Disaggregation of Revenue [Line Items]
Revenues	18,534		59,725
APAC
Disaggregation of Revenue [Line Items]
Revenues	13,340		35,158
North America
Disaggregation of Revenue [Line Items]
Segment revenues	280,838	206,389	769,137	578,717
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	194		832
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	280,644		768,305
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	256,549		704,612
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	23,241		61,716
North America | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	798		2,098
North America | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	250		711
North America | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	30,488		79,176
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	45,690		133,481
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	68,572		193,672
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	64,152		181,021
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	39,550		99,893
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	32,386		81,894
North America | North America
Disaggregation of Revenue [Line Items]
Segment revenues	271,551		747,894
North America | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	5,408		11,234
North America | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	2,208		6,300
North America | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	1,671		3,709
Europe
Disaggregation of Revenue [Line Items]
Segment revenues	171,482	157,446	516,320	431,472
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	289		1,351
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	171,193		514,969
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	155,797		471,900
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	15,001		42,035
Europe | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	173		1,119
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	511		1,266
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	61,713		190,027
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	53,634		155,208
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	19,035		59,186
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	17,650		54,637
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	5,078		15,550
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	14,372		41,712
Europe | North America
Disaggregation of Revenue [Line Items]
Segment revenues	12,536		40,074
Europe | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	146,990		444,468
Europe | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	142		233
Europe | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	11,814		31,545
Russia
Disaggregation of Revenue [Line Items]
Segment revenues	16,208	14,455	53,374	41,825
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	0		10
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	16,208		53,364
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	9,441		29,302
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	6,759		24,038
Russia | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	1		12
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	7		22
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	12,786		43,102
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	1,891		5,356
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	569		1,957
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	0		0
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	12		12
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	950		2,947
Russia | North America
Disaggregation of Revenue [Line Items]
Segment revenues	17		46
Russia | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	3		45
Russia | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	16,184		53,192
Russia | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	4		91
Total Segments
Disaggregation of Revenue [Line Items]
Segment revenues	468,528	378,290	1,338,831	1,052,014
Total Segments | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	483		2,193
Total Segments | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	468,045		1,336,638
Total Segments | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	421,787		1,205,814
Total Segments | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	45,001		127,789
Total Segments | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	972		3,229
Total Segments | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	768		1,999
Total Segments | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	104,987		312,305
Total Segments | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	101,215		294,045
Total Segments | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	88,176		254,815
Total Segments | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	81,802		235,658
Total Segments | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	44,640		115,455
Total Segments | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	47,708		126,553
Total Segments | North America
Disaggregation of Revenue [Line Items]
Segment revenues	284,104		788,014
Total Segments | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	152,401		455,747
Total Segments | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	18,534		59,725
Total Segments | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	13,489		35,345
Other Income Included in Segment Revenues
Disaggregation of Revenue [Line Items]
Segment revenues	(342)	(767)	(850)	(863)
Other income included in segment revenues	(342)	$ (767)	(850)	$ (863)
Other Income Included in Segment Revenues | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(342)		(850)
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	(342)		(850)
Other Income Included in Segment Revenues | Financial Services
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(189)		(697)
Other Income Included in Segment Revenues | Travel & Consumer
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(122)		(122)
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	0		0
Other Income Included in Segment Revenues | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(31)		(31)
Other Income Included in Segment Revenues | Emerging Verticals
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0		0
Other Income Included in Segment Revenues | North America
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(27)		(40)
Other Income Included in Segment Revenues | Europe
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(166)		(623)
Other Income Included in Segment Revenues | CIS
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0		0
Other Income Included in Segment Revenues | APAC
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	$ (149)		$ (187)