REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 1,210,941	$ 1,171,614
Revenues from performance obligations satisfied in previous period	7,200
Increase (decrease) in contract with customer, liability		6,000
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,207,255	1,168,848
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	3,686	2,766
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,074,979	1,023,153
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	130,505	143,563
Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	5,457	4,898
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	278,360	265,319
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	270,665	260,063
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	202,380	194,303
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	189,191	189,483
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	111,258	123,803
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	159,087	138,643
Americas
Disaggregation of Revenue [Line Items]
Revenues	709,802	686,793
EMEA
Disaggregation of Revenue [Line Items]
Revenues	463,937	421,950
APAC
Disaggregation of Revenue [Line Items]
Revenues	26,810	29,582
CEE
Disaggregation of Revenue [Line Items]
Revenues	10,392	33,289
North America
Disaggregation of Revenue [Line Items]
Revenues	709,957	687,711
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	707,034	685,109
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,923	2,602
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	644,628	626,807
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	61,231	56,765
North America | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	4,098	4,139
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	123,659	122,247
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	143,850	120,335
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	116,158	110,946
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	149,798	156,282
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	96,597	111,371
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	79,895	66,530
North America | Americas
Disaggregation of Revenue [Line Items]
Revenues	683,553	659,776
North America | EMEA
Disaggregation of Revenue [Line Items]
Revenues	25,372	25,098
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	726	783
North America | CEE
Disaggregation of Revenue [Line Items]
Revenues	306	2,054
Europe
Disaggregation of Revenue [Line Items]
Revenues	493,147	451,970
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	492,397	451,848
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	750	122
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	425,598	377,655
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	66,203	73,608
Europe | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,346	707
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	153,055	136,788
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	123,608	117,330
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	86,108	82,901
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	38,682	32,454
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	14,526	12,282
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	77,168	70,215
Europe | Americas
Disaggregation of Revenue [Line Items]
Revenues	25,954	25,960
Europe | EMEA
Disaggregation of Revenue [Line Items]
Revenues	438,565	396,753
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	26,084	28,799
Europe | CEE
Disaggregation of Revenue [Line Items]
Revenues	2,544	458
Russia
Disaggregation of Revenue [Line Items]
Revenues	7,837	31,933
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	7,824	31,891
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	13	42
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	4,753	18,691
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	3,071	13,190
Russia | Licensing and other revenues
Disaggregation of Revenue [Line Items]
Revenues	13	52
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	1,646	6,284
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	3,207	22,398
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	114	456
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	711	747
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	135	150
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	2,024	1,898
Russia | Americas
Disaggregation of Revenue [Line Items]
Revenues	295	1,057
Russia | EMEA
Disaggregation of Revenue [Line Items]
Revenues	0	99
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	0	0
Russia | CEE
Disaggregation of Revenue [Line Items]
Revenues	$ 7,542	$ 30,777