REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 445,647	$ 348,977	$ 869,795	$ 673,628
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	597		1,202
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	445,050		868,593
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	400,149		784,027
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	44,171		82,788
Licensing
Disaggregation of Revenue [Line Items]
Revenues	966		2,257
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	361		723
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	102,988		206,810
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	101,016		192,830
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	85,017		166,639
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	77,649		153,856
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	38,525		70,815
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	40,452		78,845
North America
Disaggregation of Revenue [Line Items]
Revenues	264,451		503,897
Europe
Disaggregation of Revenue [Line Items]
Revenues	149,804		302,889
CIS
Disaggregation of Revenue [Line Items]
Revenues	19,411		41,191
APAC
Disaggregation of Revenue [Line Items]
Revenues	11,981		21,818
North America
Disaggregation of Revenue [Line Items]
Revenues	257,103	194,025	488,299	372,328
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	294		638
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	256,809		487,661
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	233,747		448,063
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	22,483		38,475
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	638		1,300
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	235		461
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	26,732		48,688
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	47,104		87,791
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	64,536		125,100
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	59,532		116,869
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	32,876		60,343
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	26,323		49,508
North America | North America
Disaggregation of Revenue [Line Items]
Revenues	250,273		476,343
North America | Europe
Disaggregation of Revenue [Line Items]
Revenues	3,086		5,826
North America | CIS
Disaggregation of Revenue [Line Items]
Revenues	2,089		4,092
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	1,655		2,038
Europe
Disaggregation of Revenue [Line Items]
Revenues	171,495	141,319	344,838	274,026
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	634		1,062
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	170,861		343,776
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	156,637		316,103
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	14,081		27,034
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	327		946
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	450		755
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	62,496		128,314
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	52,120		101,574
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	19,857		40,151
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	18,117		36,987
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	5,649		10,472
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	13,256		27,340
Europe | North America
Disaggregation of Revenue [Line Items]
Revenues	14,177		27,538
Europe | Europe
Disaggregation of Revenue [Line Items]
Revenues	146,998		297,478
Europe | CIS
Disaggregation of Revenue [Line Items]
Revenues	28		91
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	10,292		19,731
Russia
Disaggregation of Revenue [Line Items]
Revenues	17,380	13,677	37,166	27,370
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	0		10
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	17,380		37,156
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	9,765		19,861
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	7,607		17,279
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	1		11
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	7		15
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	14,091		30,316
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	1,792		3,465
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	624		1,388
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	0		0
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	0		0
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	873		1,997
Russia | North America
Disaggregation of Revenue [Line Items]
Revenues	14		29
Russia | Europe
Disaggregation of Revenue [Line Items]
Revenues	0		42
Russia | CIS
Disaggregation of Revenue [Line Items]
Revenues	17,294		37,008
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	72		87
Total Segments
Disaggregation of Revenue [Line Items]
Revenues	445,978	$ 349,021	870,303	$ 673,724
Total Segments | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	928		1,710
Total Segments | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	445,050		868,593
Total Segments | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	400,149		784,027
Total Segments | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	44,171		82,788
Total Segments | Licensing
Disaggregation of Revenue [Line Items]
Revenues	966		2,257
Total Segments | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	692		1,231
Total Segments | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	103,319		207,318
Total Segments | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	101,016		192,830
Total Segments | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	85,017		166,639
Total Segments | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	77,649		153,856
Total Segments | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	38,525		70,815
Total Segments | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	40,452		78,845
Total Segments | North America
Disaggregation of Revenue [Line Items]
Revenues	264,464		503,910
Total Segments | Europe
Disaggregation of Revenue [Line Items]
Revenues	150,084		303,346
Total Segments | CIS
Disaggregation of Revenue [Line Items]
Revenues	19,411		41,191
Total Segments | APAC
Disaggregation of Revenue [Line Items]
Revenues	12,019		21,856
Other Income Included in Segment Revenues
Disaggregation of Revenue [Line Items]
Revenues	(331)		(508)
Other Income Included in Segment Revenues | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	(331)		(508)
Other Income Included in Segment Revenues | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Licensing
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	(331)		(508)
Other Income Included in Segment Revenues | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	(331)		(508)
Other Income Included in Segment Revenues | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | North America
Disaggregation of Revenue [Line Items]
Revenues	(13)		(13)
Other Income Included in Segment Revenues | Europe
Disaggregation of Revenue [Line Items]
Revenues	(280)		(457)
Other Income Included in Segment Revenues | CIS
Disaggregation of Revenue [Line Items]
Revenues	0		0
Other Income Included in Segment Revenues | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ (38)		$ (38)