NATURE OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Risks and Uncertainties) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Revenues	$ 1,450,448	$ 1,160,132	$ 914,128
Customer Concentration Risk \| Sales Revenue, Net \| Top Five Customers
Concentration Risk [Line Items]
Concentration percentage	24.00%	28.40%	32.80%
Revenues	$ 348,219	$ 329,324	$ 299,655
Customer Concentration Risk \| Sales Revenue, Net \| Top Ten Customers
Concentration Risk [Line Items]
Concentration percentage	33.90%	38.40%	44.10%
Revenues	$ 491,742	$ 445,814	$ 403,052
CIS Countries \| Geographic Concentration Risk \| Assets, Total
Concentration Risk [Line Items]
Cash including time deposits and restricted cash	240,148
Belarus \| Geographic Concentration Risk \| Assets, Total
Concentration Risk [Line Items]
Cash including time deposits and restricted cash	$ 196,656

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 225
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-Section S99
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