Quarterly report pursuant to sections 13 or 15(d)

Off-Balance-Sheet Risk and Concentration of Credit Risk - Additional Information (Detail)

v2.4.0.6
Off-Balance-Sheet Risk and Concentration of Credit Risk - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Concentration Risk [Line Items]    
Receivables from clearing broker $ 28,677 $ 27,368