REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 652,243	$ 588,103	$ 1,935,985	$ 1,661,023
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	650,750	587,369	1,930,085	1,659,526
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,493	734	5,900	1,497
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	559,133	507,649	1,675,172	1,451,813
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	90,421	78,670	248,719	203,771
Licensing
Disaggregation of Revenue [Line Items]
Revenues	2,012	1,041	7,914	3,243
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	677	743	4,180	2,196
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	140,014	105,799	418,998	298,966
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	136,715	130,337	396,333	364,003
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	118,854	108,384	360,188	313,833
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	110,174	112,382	336,974	323,822
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	74,203	66,769	212,326	180,741
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	72,283	64,432	211,166	179,658
North America
Disaggregation of Revenue [Line Items]
Revenues	389,976	358,401	1,161,742	1,009,841
Europe
Disaggregation of Revenue [Line Items]
Revenues	214,533	189,312	648,654	540,165
CIS
Disaggregation of Revenue [Line Items]
Revenues	30,039	26,501	76,977	69,766
APAC
Disaggregation of Revenue [Line Items]
Revenues	17,695	13,889	48,612	41,251
North America
Disaggregation of Revenue [Line Items]
Revenues	391,605	358,987	1,167,536	1,000,701
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	390,295	358,426	1,163,678	999,530
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,310	561	3,858	1,171
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	348,570	322,573	1,054,548	907,624
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	41,050	35,273	106,760	89,694
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	1,717	883	5,445	2,343
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	268	258	783	1,040
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	84,384	65,153	246,085	189,854
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	48,415	50,510	143,970	132,421
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	100,847	90,880	303,385	255,601
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	54,590	51,476	161,507	146,654
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	66,071	60,049	189,458	164,492
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	37,298	40,919	123,131	111,679
North America | North America
Disaggregation of Revenue [Line Items]
Revenues	378,493	349,875	1,126,970	976,841
North America | Europe
Disaggregation of Revenue [Line Items]
Revenues	11,278	6,756	32,805	16,722
North America | CIS
Disaggregation of Revenue [Line Items]
Revenues	1,433	2,349	6,223	6,093
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	401	7	1,538	1,045
Europe
Disaggregation of Revenue [Line Items]
Revenues	231,418	204,952	695,056	596,378
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	231,281	204,833	694,525	596,106
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	137	119	531	272
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	196,719	171,261	579,212	507,055
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	34,057	33,093	111,565	87,543
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	242	149	949	689
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	400	449	3,330	1,091
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	55,228	40,624	171,712	108,846
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	68,419	61,067	202,668	181,781
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	17,492	16,952	53,803	56,762
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	51,449	57,735	163,769	169,112
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	7,963	6,713	22,651	16,176
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	30,867	21,861	80,453	63,701
Europe | North America
Disaggregation of Revenue [Line Items]
Revenues	10,853	8,484	32,107	32,926
Europe | Europe
Disaggregation of Revenue [Line Items]
Revenues	203,255	182,533	615,843	523,180
Europe | CIS
Disaggregation of Revenue [Line Items]
Revenues	16	53	32	66
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	17,294	13,882	47,074	40,206
Russia
Disaggregation of Revenue [Line Items]
Revenues	29,220	24,164	73,393	63,944
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	29,174	24,110	71,882	63,890
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	46	54	1,511	54
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	13,844	13,815	41,412	37,134
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	15,314	10,304	30,394	26,534
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	53	9	1,520	211
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	9	36	67	65
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	402	22	1,201	266
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	19,881	18,760	49,695	49,801
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	515	552	3,000	1,470
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	4,135	3,171	11,698	8,056
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	169	7	217	73
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	4,118	1,652	7,582	4,278
Russia | North America
Disaggregation of Revenue [Line Items]
Revenues	630	42	2,665	74
Russia | Europe
Disaggregation of Revenue [Line Items]
Revenues	0	23	6	263
Russia | CIS
Disaggregation of Revenue [Line Items]
Revenues	28,590	24,099	70,722	63,607
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0