REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 632,383	$ 551,587	$ 1,283,742	$ 1,072,920
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	630,886	551,275	1,279,335	1,072,157
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,497	312	4,407	763
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	554,860	479,954	1,116,039	944,164
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	72,367	69,828	158,298	125,101
Licensing
Disaggregation of Revenue [Line Items]
Revenues	2,512	962	5,902	2,202
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	2,644	843	3,503	1,453
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	140,258	98,135	278,984	193,167
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	127,975	120,405	259,618	233,666
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	119,485	105,518	241,334	205,449
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	107,270	107,114	226,800	211,440
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	68,884	59,198	138,123	113,972
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	68,511	61,217	138,883	115,226
North America
Disaggregation of Revenue [Line Items]
Revenues	381,936	334,788	771,766	651,440
Europe
Disaggregation of Revenue [Line Items]
Revenues	211,064	177,345	434,121	350,853
CIS
Disaggregation of Revenue [Line Items]
Revenues	22,078	25,091	46,938	43,265
APAC
Disaggregation of Revenue [Line Items]
Revenues	17,305	14,363	30,917	27,362
North America
Disaggregation of Revenue [Line Items]
Revenues	384,829	331,025	775,931	641,714
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	383,573	330,819	773,383	641,104
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,256	206	2,548	610
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	348,715	300,326	705,978	585,051
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	33,855	29,681	65,710	54,421
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	1,958	624	3,728	1,460
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	301	394	515	782
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	81,481	62,340	161,701	124,701
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	47,687	43,517	95,555	81,911
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	100,801	85,600	202,538	164,721
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	52,193	48,178	106,917	95,178
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	61,776	54,287	123,387	104,443
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	40,891	37,103	85,833	70,760
North America | North America
Disaggregation of Revenue [Line Items]
Revenues	370,693	323,221	748,477	626,966
North America | Europe
Disaggregation of Revenue [Line Items]
Revenues	11,195	5,219	21,527	9,966
North America | CIS
Disaggregation of Revenue [Line Items]
Revenues	2,236	2,005	4,790	3,744
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	705	580	1,137	1,038
Europe
Disaggregation of Revenue [Line Items]
Revenues	227,305	197,361	463,638	391,426
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	227,073	197,255	463,244	391,273
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	232	106	394	153
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	192,173	167,581	382,493	335,794
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	32,253	29,204	77,508	54,450
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	545	142	707	540
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	2,334	434	2,930	642
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	58,338	35,691	116,484	68,222
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	66,655	58,879	134,249	120,714
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	18,165	19,440	36,311	39,810
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	51,253	56,175	112,320	111,377
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	7,069	4,907	14,688	9,463
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	25,825	22,269	49,586	41,840
Europe | North America
Disaggregation of Revenue [Line Items]
Revenues	10,835	11,551	21,254	24,442
Europe | Europe
Disaggregation of Revenue [Line Items]
Revenues	199,866	172,027	412,588	340,647
Europe | CIS
Disaggregation of Revenue [Line Items]
Revenues	4	0	16	13
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	16,600	13,783	29,780	26,324
Russia
Disaggregation of Revenue [Line Items]
Revenues	20,249	23,201	44,173	39,780
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	20,240	23,201	42,708	39,780
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	9	0	1,465	0
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	13,972	12,047	27,568	23,319
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	6,259	10,943	15,080	16,230
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	9	196	1,467	202
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	9	15	58	29
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	439	104	799	244
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	13,633	18,009	29,814	31,041
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	519	478	2,485	918
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	3,824	2,761	7,563	4,885
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	39	4	48	66
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	1,795	1,845	3,464	2,626
Russia | North America
Disaggregation of Revenue [Line Items]
Revenues	408	16	2,035	32
Russia | Europe
Disaggregation of Revenue [Line Items]
Revenues	3	99	6	240
Russia | CIS
Disaggregation of Revenue [Line Items]
Revenues	19,838	23,086	42,132	39,508
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0