REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 1,194,861	$ 881,366	$ 2,366,475	$ 1,662,141
Revenues from performance obligations satisfied in previous period	12,500		6,600
Increase (decrease) in contract with customer, liability		11,600		16,500
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	1,193,398	878,440	2,362,246	1,656,735
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,463	2,926	4,229	5,406
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	1,059,571	752,513	2,082,724	1,417,518
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	131,180	123,750	274,743	235,032
Licensing
Disaggregation of Revenue [Line Items]
Revenues	3,024	4,367	6,856	8,037
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	1,086	736	2,152	1,554
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	276,318	171,545	541,637	310,556
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	250,983	193,938	511,046	362,796
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	197,870	157,847	392,173	305,596
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	195,323	159,162	384,806	306,226
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	128,492	91,696	252,295	182,811
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	145,875	107,178	284,518	194,156
Americas
Disaggregation of Revenue [Line Items]
Revenues	721,612	527,398	1,408,405	997,719
EMEA
Disaggregation of Revenue [Line Items]
Revenues	422,986	291,281	844,936	550,653
CEE
Disaggregation of Revenue [Line Items]
Revenues	30,176	24,984	59,758	45,932
APAC
Disaggregation of Revenue [Line Items]
Revenues	20,087	37,703	53,376	67,837
North America
Disaggregation of Revenue [Line Items]
Revenues	723,657	532,022	1,411,368	1,006,875
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	722,311	529,230	1,407,420	1,001,604
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,346	2,792	3,948	5,271
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	654,627	466,204	1,281,434	884,091
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	65,992	61,383	122,757	114,398
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	2,463	4,076	6,098	7,611
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	575	359	1,079	775
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	130,932	85,075	253,179	154,504
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	126,879	83,342	247,214	153,082
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	114,026	92,379	224,972	179,584
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	161,034	134,638	317,316	260,224
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	115,899	80,712	227,270	156,301
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	74,887	55,876	141,417	103,180
North America | Americas
Disaggregation of Revenue [Line Items]
Revenues	697,961	508,330	1,357,737	963,097
North America | EMEA
Disaggregation of Revenue [Line Items]
Revenues	23,190	21,615	48,288	39,370
North America | CEE
Disaggregation of Revenue [Line Items]
Revenues	793	667	1,576	1,142
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	1,713	1,410	3,767	3,266
Europe
Disaggregation of Revenue [Line Items]
Revenues	451,944	311,949	903,914	588,653
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	451,847	311,840	903,695	588,571
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	97	109	219	82
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	390,296	267,070	767,951	499,761
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	60,607	44,252	134,215	87,818
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	530	260	675	314
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	511	367	1,073	760
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	140,081	79,731	276,869	144,355
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	112,766	85,965	230,096	164,005
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	83,681	65,032	166,582	125,189
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	33,847	23,934	66,301	44,907
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	12,527	10,780	24,809	26,131
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	69,042	46,507	139,257	84,066
Europe | Americas
Disaggregation of Revenue [Line Items]
Revenues	22,816	18,036	48,776	32,699
Europe | EMEA
Disaggregation of Revenue [Line Items]
Revenues	399,796	269,596	796,549	511,150
Europe | CEE
Disaggregation of Revenue [Line Items]
Revenues	29,383	24,317	58,182	44,790
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	(51)	0	407	14
Russia
Disaggregation of Revenue [Line Items]
Revenues	19,260	37,395	51,193	66,613
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	19,240	37,370	51,131	66,560
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	20	25	62	53
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	14,648	19,239	33,339	33,666
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	4,581	18,115	17,771	32,816
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	31	31	83	112
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	0	10	0	19
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	5,305	6,739	11,589	11,697
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	11,338	24,631	33,736	45,709
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	163	436	619	823
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	442	590	1,189	1,095
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	66	204	216	379
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	1,946	4,795	3,844	6,910
Russia | Americas
Disaggregation of Revenue [Line Items]
Revenues	835	1,032	1,892	1,923
Russia | EMEA
Disaggregation of Revenue [Line Items]
Revenues	0	70	99	133
Russia | CEE
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ 18,425	$ 36,293	$ 49,202	$ 64,557