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Summary of Significant Accounting Policies (Details) (USD $)
12 Months Ended
Dec. 31, 2011
criteria
party
M
Dec. 31, 2010
Dec. 31, 2009
Revenue recognition      
Number of revenue recognition criteria 4    
Cash and Cash Equivalents      
Maximum original maturity period of cash equivalents (in months) 3    
Securities Borrowed and Loaned      
Number of parties from whom matched book securities are purchased 1    
Client Commission Arrangements      
Client commissions $ 130,800,000 $ 155,800,000 $ 180,300,000
Prepaid research, gross 3,700,000 4,600,000 5,300,000
Allowance for prepaid research (400,000) (400,000) (500,000)
Prepaid research, net of allowance 3,300,000 4,200,000 4,800,000
Accrued research payables 50,721,000 41,569,000 39,000,000
Research and Development      
Research and development costs $ 47,500,000 $ 49,100,000 $ 43,100,000
Income Taxes      
Percentage of likelihood of realization that the tax position must exceed in order for the amount to be recognized 50.00%    
Maximum
     
Principles of Consolidation      
Ownership percentage for equity method investments 50.00%    
Ownership percentage for consolidation of affiliate 50.00%    
Revenue recognition      
Term of agreement (in years) 3    
Minimum
     
Principles of Consolidation      
Ownership percentage for equity method investments 20.00%    
Revenue recognition      
Term of agreement (in years) 1    
Furniture, fixtures and equipment
     
Premises and equipment      
Minimum estimated useful life (in years) 3    
Maximum estimated useful life (in years) 7    
Capitalized software
     
Premises and equipment      
Estimated useful life (in years) 3