REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 1,226,920	$ 988,539	$ 3,593,395	$ 2,650,680
Revenues from performance obligations satisfied in previous period	12,300		7,200
Increase (decrease) in contract with customer, liability		13,300		18,000
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,222,786	986,028	3,584,609	2,642,763
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	4,134	2,511	8,786	7,917
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,084,844	851,610	3,167,568	2,269,128
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	136,415	132,415	411,158	367,447
Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	5,661	4,514	14,669	14,105
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	280,402	197,566	822,039	508,122
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	254,786	230,884	765,832	593,680
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	208,898	172,989	601,071	478,585
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	205,390	174,297	590,196	480,523
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	129,675	96,063	381,970	278,874
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	147,769	116,740	432,287	310,896
Americas
Disaggregation of Revenue [Line Items]
Revenues	747,091	591,368	2,155,496	1,589,087
EMEA
Disaggregation of Revenue [Line Items]
Revenues	438,216	323,807	1,283,152	874,460
APAC
Disaggregation of Revenue [Line Items]
Revenues	31,376	28,386	91,134	74,318
CEE
Disaggregation of Revenue [Line Items]
Revenues	10,237	44,978	63,613	112,815
North America
Disaggregation of Revenue [Line Items]
Revenues	748,383	593,862	2,159,751	1,600,737
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	746,352	591,477	2,153,772	1,593,081
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,031	2,385	5,979	7,656
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	673,927	525,372	1,955,361	1,409,463
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	69,565	64,510	192,322	178,908
North America | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	4,891	3,980	12,068	12,366
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	131,181	98,494	384,360	252,998
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	134,673	100,631	381,887	253,713
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	121,703	100,536	346,675	280,120
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	170,818	146,532	488,134	406,756
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	116,878	85,534	344,148	241,835
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	73,130	62,135	214,547	165,315
North America | Americas
Disaggregation of Revenue [Line Items]
Revenues	723,015	569,331	2,080,752	1,532,428
North America | EMEA
Disaggregation of Revenue [Line Items]
Revenues	22,621	21,968	70,909	61,338
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	1,237	1,001	2,813	2,143
North America | CEE
Disaggregation of Revenue [Line Items]
Revenues	1,510	1,562	5,277	4,828
Europe
Disaggregation of Revenue [Line Items]
Revenues	470,009	350,080	1,373,923	938,733
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	468,118	349,984	1,371,390	938,555
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,891	96	2,533	178
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	405,030	303,521	1,172,981	803,282
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	64,426	46,070	198,641	133,888
Europe | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	553	489	2,301	1,563
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	147,271	91,197	424,140	235,552
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	115,372	98,711	345,468	262,716
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	87,028	71,953	253,610	197,142
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	34,513	27,103	100,814	72,010
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	12,569	10,374	37,378	36,505
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	73,256	50,742	212,513	134,808
Europe | Americas
Disaggregation of Revenue [Line Items]
Revenues	23,575	20,959	72,351	53,658
Europe | EMEA
Disaggregation of Revenue [Line Items]
Revenues	415,595	301,736	1,212,144	812,886
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	30,139	27,385	88,321	72,175
Europe | CEE
Disaggregation of Revenue [Line Items]
Revenues	700	0	1,107	14
Russia
Disaggregation of Revenue [Line Items]
Revenues	8,528	44,597	59,721	111,210
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	8,316	44,567	59,447	111,127
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	212	30	274	83
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	5,887	22,717	39,226	56,383
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	2,424	21,835	20,195	54,651
Russia | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	217	45	300	176
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	1,950	7,875	13,539	19,572
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	4,741	31,542	38,477	77,251
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	167	500	786	1,323
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	59	662	1,248	1,757
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	228	155	444	534
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	1,383	3,863	5,227	10,773
Russia | Americas
Disaggregation of Revenue [Line Items]
Revenues	501	1,078	2,393	3,001
Russia | EMEA
Disaggregation of Revenue [Line Items]
Revenues	0	103	99	236
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Russia | CEE
Disaggregation of Revenue [Line Items]
Revenues	$ 8,027	$ 43,416	$ 57,229	$ 107,973