REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 1,171,614	$ 780,775
Revenues from performance obligations satisfied in previous period	6,000
Increase (decrease) in contract with customer, liability		15,200
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,168,848	778,295
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,766	2,480
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,023,153	665,005
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	143,563	111,282
Licensing
Disaggregation of Revenue [Line Items]
Revenues	3,832	3,670
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,066	818
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	265,319	139,011
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	260,063	168,858
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	194,303	147,749
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	189,483	147,064
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	123,803	91,115
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	138,643	86,978
Americas
Disaggregation of Revenue [Line Items]
Revenues	686,793	470,321
EMEA
Disaggregation of Revenue [Line Items]
Revenues	421,950	259,372
CEE
Disaggregation of Revenue [Line Items]
Revenues	33,289	30,134
APAC
Disaggregation of Revenue [Line Items]
Revenues	29,582	20,948
North America
Disaggregation of Revenue [Line Items]
Revenues	687,711	474,853
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	685,109	472,374
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,602	2,479
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	626,807	417,887
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	56,765	53,015
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	3,635	3,535
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	504	416
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	122,247	69,429
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	120,335	69,740
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	110,946	87,205
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	156,282	125,586
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	111,371	75,589
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	66,530	47,304
North America | Americas
Disaggregation of Revenue [Line Items]
Revenues	659,776	454,767
North America | EMEA
Disaggregation of Revenue [Line Items]
Revenues	25,098	17,755
North America | CEE
Disaggregation of Revenue [Line Items]
Revenues	2,054	1,856
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	783	475
Europe
Disaggregation of Revenue [Line Items]
Revenues	451,970	276,704
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	451,848	276,731
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	122	(27)
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	377,655	232,691
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	73,608	43,566
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	145	54
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	562	393
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	136,788	64,624
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	117,330	78,040
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	82,901	60,157
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	32,454	20,973
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	12,282	15,351
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	70,215	37,559
Europe | Americas
Disaggregation of Revenue [Line Items]
Revenues	25,960	14,663
Europe | EMEA
Disaggregation of Revenue [Line Items]
Revenues	396,753	241,554
Europe | CEE
Disaggregation of Revenue [Line Items]
Revenues	458	14
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	28,799	20,473
Russia
Disaggregation of Revenue [Line Items]
Revenues	31,933	29,218
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	31,891	29,190
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	42	28
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	18,691	14,427
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	13,190	14,701
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	52	81
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	0	9
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	6,284	4,958
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	22,398	21,078
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	456	387
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	747	505
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	150	175
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	1,898	2,115
Russia | Americas
Disaggregation of Revenue [Line Items]
Revenues	1,057	891
Russia | EMEA
Disaggregation of Revenue [Line Items]
Revenues	99	63
Russia | CEE
Disaggregation of Revenue [Line Items]
Revenues	30,777	28,264
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0