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	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 230,042	$ 234,568
Securities Purchased Under Agreements to Resell	7,075
Accounts Receivable	57,769	89,098
Receivable from Employees and Related Parties	9,210	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	18,648	17,909
Securities Sold Under Agreements to Repurchase	92,110	120,787
Payable to Employees and Related Parties	14,977	12,964
Notes Payable	127,682	136,860
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	230,042	234,568
Securities Purchased Under Agreements to Resell	7,075
Accounts Receivable	57,769	89,098
Receivable from Employees and Related Parties	9,210	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	18,648	17,909
Securities Sold Under Agreements to Repurchase	92,110	120,787
Payable to Employees and Related Parties	14,977	12,964
Notes Payable	102,748	101,375
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	230,042	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	7,075
Accounts Receivable	57,769	89,098
Receivable from Employees and Related Parties	9,210	5,166
Accounts Payable and Accrued Expenses	18,648	17,909
Securities Sold Under Agreements to Repurchase	92,110	120,787
Payable to Employees and Related Parties	14,977	12,964
Notes Payable	$ 127,682	$ 136,860