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Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Accounting Policies [Abstract]        
Dividend Income $ 9,009 $ 288 $ 11,484 $ 1,049
Dividend Expense $ (6,899) $ (809) $ (8,035) $ (1,321)