OPERATING SEGMENTS, Revenues from External Customers and Segment Operating Profit Before Unallocated Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 200,045	$ 160,384
Operating profit	22,820	21,857
Russia
Segment Reporting Information [Line Items]
Revenues	7,259	10,390
Operating Segments
Segment Reporting Information [Line Items]
Revenues	200,194	159,997
Operating profit	38,680	29,879
Operating Segments | North America
Segment Reporting Information [Line Items]
Revenues	100,269	80,198
Operating profit	24,068	18,197
Operating Segments | Europe
Segment Reporting Information [Line Items]
Revenues	91,152	67,659
Operating profit	15,340	14,135
Operating Segments | Russia
Segment Reporting Information [Line Items]
Revenues	7,513	10,748
Operating profit	(271)	(1,135)
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	1,260	1,392
Operating profit	(457)	(1,318)
Accounts Receivable | Customer Concentration Risk | Billed | UBS AG
Segment Reporting Information [Line Items]
Number of customers	1
Percentage of total revenues	15.60%
Accounts Receivable | Customer Concentration Risk | Unbilled | UBS AG and Barclays Capital
Segment Reporting Information [Line Items]
Number of customers	2
Percentage of total revenues	27.10%
Segment Revenues | Customer Concentration Risk | UBS AG
Segment Reporting Information [Line Items]
Number of customers	1	1
Segment Revenues | Customer Concentration Risk | Operating Segments | UBS AG
Segment Reporting Information [Line Items]
Revenues	$ 30,933	$ 20,024