REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 3,758,144	$ 2,659,478	$ 2,293,798
Revenues from performance obligations satisfied in previous period	18,700	5,000	7,800
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	3,747,565	2,650,991	2,291,245
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	10,579	8,487	2,553
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	3,209,747	2,291,004	1,990,653
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	529,396	352,012	294,650
Licensing
Disaggregation of Revenue [Line Items]
Revenues	15,552	11,139	5,081
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	3,449	5,323	3,414
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	848,370	555,235	500,872
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	741,128	458,789	439,358
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	666,941	560,680	420,923
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	664,597	496,813	433,398
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	391,309	296,313	248,452
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	445,799	291,648	250,795
North America
Disaggregation of Revenue [Line Items]
Revenues	2,242,248	1,601,820	1,380,944
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	2,232,308	1,595,786	1,379,256
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	9,940	6,034	1,688
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,981,696	1,440,635	1,247,979
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	244,249	151,769	127,926
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	14,540	8,027	3,626
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,763	1,389	1,413
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	361,611	199,594	184,469
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	359,306	221,977	198,264
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	389,613	334,063	262,448
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	559,707	419,895	354,023
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	340,706	260,518	224,925
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	231,305	165,773	156,815
Europe
Disaggregation of Revenue [Line Items]
Revenues	1,350,484	947,305	820,717
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,349,956	946,379	819,913
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	528	926	804
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,145,606	790,203	688,605
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	202,436	151,718	128,977
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	793	1,526	1,230
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,649	3,858	1,905
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	372,394	278,355	244,284
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	354,041	220,448	229,523
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	275,502	224,922	157,844
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	102,270	73,288	77,377
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	49,900	35,347	23,444
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	196,377	114,945	88,245
Russia
Disaggregation of Revenue [Line Items]
Revenues	165,412	110,353	92,137
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	165,301	108,826	92,076
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	111	1,527	61
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	82,445	60,166	54,069
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	82,711	48,525	37,747
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	219	1,586	225
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	37	76	96
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	114,365	77,286	72,119
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	27,781	16,364	11,571
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	1,826	1,695	631
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	2,620	3,630	1,998
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	703	448	83
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	18,117	10,930	5,735
Americas
Disaggregation of Revenue [Line Items]
Revenues	2,226,830	1,595,136	1,390,015
Americas | North America
Disaggregation of Revenue [Line Items]
Revenues	2,145,163	1,546,093	1,344,040
Americas | Europe
Disaggregation of Revenue [Line Items]
Revenues	77,351	45,553	45,859
Americas | Russia
Disaggregation of Revenue [Line Items]
Revenues	4,316	3,490	116
EMEA
Disaggregation of Revenue [Line Items]
Revenues	1,259,717	879,842	746,866
EMEA | North America
Disaggregation of Revenue [Line Items]
Revenues	87,121	45,733	27,042
EMEA | Europe
Disaggregation of Revenue [Line Items]
Revenues	1,172,267	834,033	719,548
EMEA | Russia
Disaggregation of Revenue [Line Items]
Revenues	329	76	276
CEE
Disaggregation of Revenue [Line Items]
Revenues	168,038	114,702	100,471
CEE | North America
Disaggregation of Revenue [Line Items]
Revenues	6,740	7,817	8,583
CEE | Europe
Disaggregation of Revenue [Line Items]
Revenues	531	98	143
CEE | Russia
Disaggregation of Revenue [Line Items]
Revenues	160,767	106,787	91,745
APAC
Disaggregation of Revenue [Line Items]
Revenues	103,559	69,798	56,446
APAC | North America
Disaggregation of Revenue [Line Items]
Revenues	3,224	2,177	1,279
APAC | Europe
Disaggregation of Revenue [Line Items]
Revenues	100,335	67,621	55,167
APAC | Russia
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0