REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 551,587	$ 445,647	$ 1,072,920	$ 869,795
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	312	597	763	1,202
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	551,275	445,050	1,072,157	868,593
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	479,954	400,149	944,164	784,027
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	69,828	44,171	125,101	82,788
Licensing
Disaggregation of Revenue [Line Items]
Revenues	962	966	2,202	2,257
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	843	361	1,453	723
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	120,405	102,988	233,666	206,810
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	107,114	101,016	211,440	192,830
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	105,518	85,017	205,449	166,639
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	98,135	77,649	193,167	153,856
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	59,198	38,525	113,972	70,815
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	61,217	40,452	115,226	78,845
North America
Disaggregation of Revenue [Line Items]
Revenues	334,788	264,451	651,440	503,897
Europe
Disaggregation of Revenue [Line Items]
Revenues	177,345	149,804	350,853	302,889
CIS
Disaggregation of Revenue [Line Items]
Revenues	25,091	19,411	43,265	41,191
APAC
Disaggregation of Revenue [Line Items]
Revenues	14,363	11,981	27,362	21,818
North America
Disaggregation of Revenue [Line Items]
Segment revenues	331,039	257,103	641,728	488,299
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	220	294	624	638
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	330,819	256,809	641,104	487,661
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	300,326	233,747	585,051	448,063
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	29,681	22,483	54,421	38,475
North America | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	624	638	1,460	1,300
North America | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	408	235	796	461
North America | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	43,531	26,732	81,925	48,688
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	48,178	47,104	95,178	87,791
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	85,600	64,536	164,721	125,100
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	62,340	59,532	124,701	116,869
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	54,287	32,876	104,443	60,343
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	37,103	26,323	70,760	49,508
North America | North America
Disaggregation of Revenue [Line Items]
Segment revenues	323,221	250,273	626,966	476,343
North America | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	5,233	3,086	9,980	5,826
North America | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	2,005	2,089	3,744	4,092
North America | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	580	1,655	1,038	2,038
Europe
Disaggregation of Revenue [Line Items]
Segment revenues	197,452	171,495	391,675	344,838
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	197	634	402	1,062
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	197,255	170,861	391,273	343,776
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	167,581	156,637	335,794	316,103
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	29,204	14,081	54,450	27,034
Europe | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	142	327	540	946
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	525	450	891	755
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	58,966	62,496	120,954	128,314
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	56,175	52,120	111,377	101,574
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	19,442	19,857	39,812	40,151
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	35,692	18,117	68,228	36,987
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	4,907	5,649	9,463	10,472
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	22,270	13,256	41,841	27,340
Europe | North America
Disaggregation of Revenue [Line Items]
Segment revenues	11,553	14,177	24,444	27,538
Europe | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	172,099	146,998	340,866	297,478
Europe | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	0	28	13	91
Europe | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	13,800	10,292	26,352	19,731
Russia
Disaggregation of Revenue [Line Items]
Segment revenues	23,201	17,380	39,781	37,166
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	0	0	1	10
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	23,201	17,380	39,780	37,156
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	12,047	9,765	23,319	19,861
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	10,943	7,607	16,230	17,279
Russia | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	196	1	202	11
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	15	7	30	15
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	18,009	14,091	31,041	30,316
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	2,761	1,792	4,885	3,465
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	478	624	918	1,388
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	104	0	244	0
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	4	0	66	0
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	1,845	873	2,627	1,997
Russia | North America
Disaggregation of Revenue [Line Items]
Segment revenues	16	14	32	29
Russia | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	99	0	240	42
Russia | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	23,086	17,294	39,509	37,008
Russia | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	0	72	0	87
Total Segments
Disaggregation of Revenue [Line Items]
Segment revenues	551,692	445,978	1,073,184	870,303
Total Segments | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Segment revenues	417	928	1,027	1,710
Total Segments | Transferred over time
Disaggregation of Revenue [Line Items]
Segment revenues	551,275	445,050	1,072,157	868,593
Total Segments | Time-and-material
Disaggregation of Revenue [Line Items]
Segment revenues	479,954	400,149	944,164	784,027
Total Segments | Fixed-price
Disaggregation of Revenue [Line Items]
Segment revenues	69,828	44,171	125,101	82,788
Total Segments | Licensing
Disaggregation of Revenue [Line Items]
Segment revenues	962	966	2,202	2,257
Total Segments | Other revenues
Disaggregation of Revenue [Line Items]
Segment revenues	948	692	1,717	1,231
Total Segments | Financial Services
Disaggregation of Revenue [Line Items]
Segment revenues	120,506	103,319	233,920	207,318
Total Segments | Travel & Consumer
Disaggregation of Revenue [Line Items]
Segment revenues	107,114	101,016	211,440	192,830
Total Segments | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Segment revenues	105,520	85,017	205,451	166,639
Total Segments | Business Information & Media
Disaggregation of Revenue [Line Items]
Segment revenues	98,136	77,649	193,173	153,856
Total Segments | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Segment revenues	59,198	38,525	113,972	70,815
Total Segments | Emerging Verticals
Disaggregation of Revenue [Line Items]
Segment revenues	61,218	40,452	115,228	78,845
Total Segments | North America
Disaggregation of Revenue [Line Items]
Segment revenues	334,790	264,464	651,442	503,910
Total Segments | Europe
Disaggregation of Revenue [Line Items]
Segment revenues	177,431	150,084	351,086	303,346
Total Segments | CIS
Disaggregation of Revenue [Line Items]
Segment revenues	25,091	19,411	43,266	41,191
Total Segments | APAC
Disaggregation of Revenue [Line Items]
Segment revenues	14,380	12,019	27,390	21,856
Other Income Included in Segment Revenues
Disaggregation of Revenue [Line Items]
Segment revenues	(105)	(331)	(264)	(508)
Other income included in segment revenues	(105)	(331)	(264)	(508)
Other Income Included in Segment Revenues | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(105)	(331)	(264)	(508)
Other Income Included in Segment Revenues | Transferred over time
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Time-and-material
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Fixed-price
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Licensing
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Other revenues
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(105)	(331)	(264)	(508)
Other Income Included in Segment Revenues | Financial Services
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(101)	(331)	(254)	(508)
Other Income Included in Segment Revenues | Travel & Consumer
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(2)	0	(2)	0
Other Income Included in Segment Revenues | Business Information & Media
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(1)	0	(6)	0
Other Income Included in Segment Revenues | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	0	0
Other Income Included in Segment Revenues | Emerging Verticals
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(1)	0	(2)	0
Other Income Included in Segment Revenues | North America
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(2)	(13)	(2)	(13)
Other Income Included in Segment Revenues | Europe
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	(86)	(280)	(233)	(457)
Other Income Included in Segment Revenues | CIS
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	0	0	(1)	0
Other Income Included in Segment Revenues | APAC
Disaggregation of Revenue [Line Items]
Other income included in segment revenues	$ (17)	$ (38)	$ (28)	$ (38)