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Cash and Securities Segregated For Regulatory and Other Purposes - Additional Information (Detail) (USD $)
Dec. 31, 2012
Dec. 31, 2011
Cash and Cash Equivalents [Line Items]    
Deposits held in special reserve bank accounts $ 32,000,000 $ 30,100,000
Client funds held in escrow 242,700,000 166,700,000
Oppenheimer Trust's deposit with State of Ohio $ 100,000