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Receivables from and Payables to Brokers, Dealers, and Clearing Agencies - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Broker Dealer Receivables And Payable [Line Items]      
Interest on margin payable $ 1,456 $ 2,513 $ 828