OPERATING SEGMENTS, Revenues from External Customers and Segment Operating Profit Before Unallocated Expenses (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Revenues	$ 217,781	$ 174,695	$ 417,826	$ 335,079
Operating profit	23,606	18,476	46,426	40,333
Russia
Segment Reporting Information [Line Items]
Revenues	9,798	12,772	17,057	23,162
Operating Segments
Segment Reporting Information [Line Items]
Revenues	218,069	174,683	418,263	334,680
Operating profit	44,433	32,542	83,113	62,421
Operating Segments | North America
Segment Reporting Information [Line Items]
Revenues	108,213	88,275	208,482	168,473
Operating profit	25,746	21,529	49,814	39,726
Operating Segments | Europe
Segment Reporting Information [Line Items]
Revenues	98,160	71,514	189,312	139,173
Operating profit	16,670	9,714	32,010	23,849
Operating Segments | Russia
Segment Reporting Information [Line Items]
Revenues	10,212	13,371	17,725	24,119
Operating profit	1,740	2,199	1,469	1,064
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	1,484	1,523	2,744	2,915
Operating profit	$ 277	$ (900)	$ (180)	$ (2,218)
Accounts Receivable | Customer Concentration Risk | Unbilled | UBS AG
Segment Reporting Information [Line Items]
Number of customers			1
Percentage of total revenues			14.00%
Accounts Receivable | Customer Concentration Risk | Billed | UBS AG
Segment Reporting Information [Line Items]
Number of customers			1
Percentage of total revenues			12.70%
Segment Revenues | Customer Concentration Risk | UBS AG
Segment Reporting Information [Line Items]
Number of customers	1	1	1	1
Segment Revenues | Customer Concentration Risk | Operating Segments | UBS AG
Segment Reporting Information [Line Items]
Revenues	$ 31,933	$ 23,224	$ 62,867	$ 43,247