OPERATING SEGMENTS, Revenues from External Customers and Segment Operating Profit Before Unallocated Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 174,695	$ 133,184	$ 335,079	$ 257,382
Operating profit	18,476	17,535	40,333	33,071
Entity-Wide Revenue, Major Customer, Amount	174,695	133,184	335,079	257,382
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	23,224		43,247
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		13,474		25,581
Percentage of total revenues		10.30%		9.90%
Russia
Segment Reporting Information [Line Items]
Revenues	12,772	13,192	23,162	24,930
Operating Segments
Segment Reporting Information [Line Items]
Revenues	174,683	133,174	334,680	257,350
Operating profit	32,542	25,870	62,421	49,138
Entity-Wide Revenue, Major Customer, Amount	174,683	133,174	334,680	257,350
Operating Segments | North America
Segment Reporting Information [Line Items]
Revenues	88,275	69,076	168,473	132,133
Operating profit	21,529	16,213	39,726	30,394
Operating Segments | Europe
Segment Reporting Information [Line Items]
Revenues	71,514	48,424	139,173	94,577
Operating profit	9,714	8,118	23,849	16,489
Operating Segments | Russia
Segment Reporting Information [Line Items]
Revenues	13,371	13,498	24,119	25,851
Operating profit	2,199	1,687	1,064	2,319
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenues	1,523	2,176	2,915	4,789
Operating profit	$ (900)	$ (148)	$ (2,218)	$ (64)