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	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 122,435	$ 234,568
Securities Purchased Under Agreements to Resell	3,879
Accounts Receivable	114,243	89,098
Receivable from Employees and Related Parties	9,111	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	20,968	17,909
Securities Sold Under Agreements to Repurchase	82,930	120,787
Payable to Employees and Related Parties	16,772	12,964
Notes Payable	128,381	136,860
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	122,435	234,568
Securities Purchased Under Agreements to Resell	3,879
Accounts Receivable	114,243	89,098
Receivable from Employees and Related Parties	9,111	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	20,968	17,909
Securities Sold Under Agreements to Repurchase	82,930	120,787
Payable to Employees and Related Parties	16,772	12,964
Notes Payable	102,282	101,375
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	122,435	234,568
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Purchased Under Agreements to Resell	3,879
Accounts Receivable	114,243	89,098
Receivable from Employees and Related Parties	9,111	5,166
Accounts Payable and Accrued Expenses	20,968	17,909
Securities Sold Under Agreements to Repurchase	82,930	120,787
Payable to Employees and Related Parties	16,772	12,964
Notes Payable	$ 128,381	$ 136,860