OPERATING SEGMENTS, Revenues from External Customers and Segment Operating Profit Before Unallocated Expenses (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Revenues	$ 236,049	$ 192,764	$ 653,875	$ 527,843
Operating profit	27,776	21,835	74,202	62,168
Russia
Segment Reporting Information [Line Items]
Revenues	9,469	14,351	26,526	37,513
Operating Segments
Segment Reporting Information [Line Items]
Revenues	236,174	192,759	654,437	527,439
Operating profit	47,721	37,028	130,834	99,449
Operating Segments | North America
Segment Reporting Information [Line Items]
Revenues	121,746	99,393	330,228	267,866
Operating profit	28,906	22,557	78,720	62,283
Operating Segments | Europe
Segment Reporting Information [Line Items]
Revenues	103,514	78,074	292,826	217,247
Operating profit	17,021	12,200	49,031	36,049
Operating Segments | Russia
Segment Reporting Information [Line Items]
Revenues	9,707	14,762	27,432	38,881
Operating profit	1,626	3,779	3,095	4,843
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	1,207	530	3,951	3,445
Operating profit	$ 168	$ (1,508)	$ (12)	$ (3,726)
Accounts Receivable | Customer Concentration Risk | Unbilled | UBS AG
Segment Reporting Information [Line Items]
Number of customers			1
Percentage of total revenues			16.50%
Accounts Receivable | Customer Concentration Risk | Billed | UBS AG
Segment Reporting Information [Line Items]
Number of customers			0
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	$ 32,960	$ 27,012	$ 95,826	$ 70,259