Carrying Amount and Estimated Fair Value of Financial Instrument Assets and Liabilities which are Not Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 111,179	$ 234,568
Securities Purchased Under Agreements to Resell	9,313
Accounts Receivable	77,987	89,098
Receivable from Employees and Related Parties	10,244	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	16,580	17,909
Securities Sold Under Agreements to Repurchase	73,325	120,787
Payable to Employees and Related Parties	15,813	12,964
Notes Payable	134,033	136,860
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	111,179	234,568
Securities Purchased Under Agreements to Resell	9,313
Accounts Receivable	77,987	89,098
Receivable from Employees and Related Parties	10,244	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	16,580	17,909
Securities Sold Under Agreements to Repurchase	73,325	120,787
Payable to Employees and Related Parties	15,813	12,964
Notes Payable	101,823	101,375
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	111,179	234,568
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Purchased Under Agreements to Resell	9,313
Accounts Receivable	77,987	89,098
Receivable from Employees and Related Parties	10,244	5,166
Accounts Payable and Accrued Expenses	16,580	17,909
Securities Sold Under Agreements to Repurchase	73,325	120,787
Payable to Employees and Related Parties	15,813	12,964
Notes Payable	$ 134,033	$ 136,860