REVENUES (Disaggregation of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 651,359	$ 521,333
Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	2,910	451
Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	648,449	520,882
Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	561,179	464,210
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	85,931	55,273
Licensing
Disaggregation of Revenue [Line Items]
Revenues	3,390	1,240
Other revenues
Disaggregation of Revenue [Line Items]
Revenues	859	610
Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	138,726	95,032
Financial Services
Disaggregation of Revenue [Line Items]
Revenues	131,643	113,261
Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	121,849	99,931
Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	119,530	104,326
Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	69,239	54,774
Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	70,372	54,009
North America
Disaggregation of Revenue [Line Items]
Revenues	389,830	316,652
Europe
Disaggregation of Revenue [Line Items]
Revenues	223,057	173,508
CIS
Disaggregation of Revenue [Line Items]
Revenues	24,860	18,174
APAC
Disaggregation of Revenue [Line Items]
Revenues	13,612	12,999
North America
Disaggregation of Revenue [Line Items]
Revenues	391,102	310,689
North America | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,292	404
North America | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	389,810	310,285
North America | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	357,263	284,725
North America | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	31,855	24,740
North America | Licensing
Disaggregation of Revenue [Line Items]
Revenues	1,770	836
North America | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	214	388
North America | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	80,220	62,361
North America | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	47,868	38,394
North America | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	101,737	79,121
North America | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	54,724	47,000
North America | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	61,611	50,156
North America | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	44,942	33,657
North America | North America
Disaggregation of Revenue [Line Items]
Revenues	377,784	303,745
North America | Europe
Disaggregation of Revenue [Line Items]
Revenues	10,332	4,747
North America | CIS
Disaggregation of Revenue [Line Items]
Revenues	2,554	1,739
North America | APAC
Disaggregation of Revenue [Line Items]
Revenues	432	458
Europe
Disaggregation of Revenue [Line Items]
Revenues	236,333	194,065
Europe | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	162	47
Europe | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	236,171	194,018
Europe | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	190,320	168,213
Europe | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	45,255	25,246
Europe | Licensing
Disaggregation of Revenue [Line Items]
Revenues	162	398
Europe | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	596	208
Europe | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	58,146	32,531
Europe | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	67,594	61,835
Europe | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	18,146	20,370
Europe | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	61,067	55,202
Europe | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	7,619	4,556
Europe | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	23,761	19,571
Europe | North America
Disaggregation of Revenue [Line Items]
Revenues	10,419	12,891
Europe | Europe
Disaggregation of Revenue [Line Items]
Revenues	212,722	168,620
Europe | CIS
Disaggregation of Revenue [Line Items]
Revenues	12	13
Europe | APAC
Disaggregation of Revenue [Line Items]
Revenues	13,180	12,541
Russia
Disaggregation of Revenue [Line Items]
Revenues	23,924	16,579
Russia | Transferred at a point of time
Disaggregation of Revenue [Line Items]
Revenues	1,456	0
Russia | Transferred over time
Disaggregation of Revenue [Line Items]
Revenues	22,468	16,579
Russia | Time-and-material
Disaggregation of Revenue [Line Items]
Revenues	13,596	11,272
Russia | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	8,821	5,287
Russia | Licensing
Disaggregation of Revenue [Line Items]
Revenues	1,458	6
Russia | Other revenues
Disaggregation of Revenue [Line Items]
Revenues	49	14
Russia | Business Information & Media
Disaggregation of Revenue [Line Items]
Revenues	360	140
Russia | Financial Services
Disaggregation of Revenue [Line Items]
Revenues	16,181	13,032
Russia | Software & Hi-Tech
Disaggregation of Revenue [Line Items]
Revenues	1,966	440
Russia | Travel & Consumer
Disaggregation of Revenue [Line Items]
Revenues	3,739	2,124
Russia | Life Sciences & Healthcare
Disaggregation of Revenue [Line Items]
Revenues	9	62
Russia | Emerging Verticals
Disaggregation of Revenue [Line Items]
Revenues	1,669	781
Russia | North America
Disaggregation of Revenue [Line Items]
Revenues	1,627	16
Russia | Europe
Disaggregation of Revenue [Line Items]
Revenues	3	141
Russia | CIS
Disaggregation of Revenue [Line Items]
Revenues	22,294	16,422
Russia | APAC
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0