REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 1,152,136	$ 1,226,920	$ 3,533,283	$ 3,593,395
Revenues from performance obligations satisfied in previous period	9,200		7,100
Increase (decrease) in contract with customer, liability		12,300		7,200
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,149,241	1,222,786	3,522,805	3,584,609
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,895	4,134	10,478	8,786
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	995,599	1,084,844	3,098,598	3,167,568
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	147,056	136,415	412,914	411,158
Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	9,481	5,661	21,771	14,669
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	262,957	280,402	814,776	822,039
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	246,435	254,786	776,068	765,832
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	183,861	208,898	575,925	601,071
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	174,346	205,390	538,755	590,196
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	124,244	129,675	349,943	381,970
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	160,293	147,769	477,816	432,287
Americas
Disaggregation of Revenue [Line Items]
Revenues	677,410	747,091	2,066,299	2,155,496
EMEA
Disaggregation of Revenue [Line Items]
Revenues	446,237	438,216	1,369,259	1,283,152
APAC
Disaggregation of Revenue [Line Items]
Revenues	25,051	31,376	76,080	91,134
CEE
Disaggregation of Revenue [Line Items]
Revenues	3,438	10,237	21,645	63,613
North America
Disaggregation of Revenue [Line Items]
Revenues	684,778	748,383	2,074,806	2,159,751
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	682,410	746,352	2,066,081	2,153,772
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,368	2,031	8,725	5,979
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	602,765	673,927	1,854,159	1,955,361
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	73,640	69,565	202,144	192,322
North America | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	8,373	4,891	18,503	12,068
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	115,778	131,181	359,475	384,360
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	131,499	134,673	410,527	381,887
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	104,863	121,703	327,378	346,675
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	136,520	170,818	422,073	488,134
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	109,137	116,878	305,700	344,148
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	86,981	73,130	249,653	214,547
North America | Americas
Disaggregation of Revenue [Line Items]
Revenues	654,717	723,015	1,991,871	2,080,752
North America | EMEA
Disaggregation of Revenue [Line Items]
Revenues	29,251	22,621	80,252	70,909
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	786	1,237	2,161	2,813
North America | CEE
Disaggregation of Revenue [Line Items]
Revenues	24	1,510	522	5,277
Europe
Disaggregation of Revenue [Line Items]
Revenues	465,813	470,009	1,442,402	1,373,923
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	465,286	468,118	1,440,682	1,371,390
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	527	1,891	1,720	2,533
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	391,479	405,030	1,233,271	1,172,981
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	73,226	64,426	205,897	198,641
Europe | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,108	553	3,234	2,301
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	146,837	147,271	451,531	424,140
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	114,122	115,372	358,091	345,468
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	78,983	87,028	248,351	253,610
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	37,545	34,513	115,137	100,814
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	15,143	12,569	44,123	37,378
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	73,183	73,256	225,169	212,513
Europe | Americas
Disaggregation of Revenue [Line Items]
Revenues	22,693	23,575	73,797	72,351
Europe | EMEA
Disaggregation of Revenue [Line Items]
Revenues	416,986	415,595	1,289,007	1,212,144
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	24,265	30,139	73,919	88,321
Europe | CEE
Disaggregation of Revenue [Line Items]
Revenues	1,869	700	5,679	1,107
Russia
Disaggregation of Revenue [Line Items]
Revenues	1,545	8,528	16,075	59,721
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,545	8,316	16,042	59,447
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	0	212	33	274
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,355	5,887	11,168	39,226
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	190	2,424	4,873	20,195
Russia | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	0	217	34	300
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	342	1,950	3,770	13,539
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	814	4,741	7,450	38,477
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	15	167	196	786
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	281	59	1,545	1,248
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	(36)	228	120	444
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	129	1,383	2,994	5,227
Russia | Americas
Disaggregation of Revenue [Line Items]
Revenues	0	501	631	2,393
Russia | EMEA
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	99
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Russia | CEE
Disaggregation of Revenue [Line Items]
Revenues	$ 1,545	$ 8,027	$ 15,444	$ 57,229