OPERATING SEGMENTS, Revenues from External Customers and Segment Operating Profit Before Unallocated Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 192,764	$ 140,150	$ 527,843	$ 397,532
Operating profit	21,835	20,230	62,168	53,301
Entity-wide revenue, major customer, amount	192,764	140,150	527,843	397,532
UBS AG
Segment Reporting Information [Line Items]
Number of customers accounted for more than ten percentage of revenue	1		1
Entity-wide revenue, major customer, amount	27,012		70,259
Barclays Capital
Segment Reporting Information [Line Items]
Number of customers accounted for more than ten percentage of revenue		1		1
Entity-wide revenue, major customer, amount		14,489		40,070
Russia
Segment Reporting Information [Line Items]
Revenues	14,351	11,534	37,513	36,464
Operating Segments
Segment Reporting Information [Line Items]
Revenues	192,759	140,139	527,439	397,489
Operating profit	37,028	27,597	99,449	76,735
Entity-wide revenue, major customer, amount	192,759	140,139	527,439	397,489
Operating Segments | North America
Segment Reporting Information [Line Items]
Revenues	99,393	73,480	267,866	205,613
Operating profit	22,557	18,133	62,283	48,527
Operating Segments | Europe
Segment Reporting Information [Line Items]
Revenues	78,074	51,531	217,247	146,108
Operating profit	12,200	8,049	36,049	24,538
Operating Segments | Russia
Segment Reporting Information [Line Items]
Revenues	14,762	12,050	38,881	37,901
Operating profit	3,779	391	4,843	2,710
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	530	3,078	3,445	7,867
Operating profit	$ (1,508)	$ 1,024	$ (3,726)	$ 960
Accounts Receivable | Customer Concentration Risk | Billed | UBS AG
Segment Reporting Information [Line Items]
Percentage of total revenues			10.00%
Accounts Receivable | Customer Concentration Risk | Unbilled | UBS AG
Segment Reporting Information [Line Items]
Percentage of total revenues			15.50%