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	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 234,568	$ 136,614
Securities Purchased Under Agreements to Resell		2,146
Accounts Receivable	89,098	52,060
Receivable from Employees and Related Parties	5,166	7,793
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	17,909	17,849
Securities Sold Under Agreements to Repurchase	120,787	129,577
Payable to Employees and Related Parties	12,964	12,647
Notes Payable	136,860	122,279
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	234,568	136,614
Securities Purchased Under Agreements to Resell		2,146
Accounts Receivable	89,098	52,060
Receivable from Employees and Related Parties	5,166	7,793
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	17,909	17,849
Securities Sold Under Agreements to Repurchase	120,787	129,577
Payable to Employees and Related Parties	12,964	12,647
Notes Payable	101,375	99,664
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	234,568	136,614
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		2,146
Accounts Receivable	89,098	52,060
Receivable from Employees and Related Parties	5,166	7,793
Accounts Payable and Accrued Expenses	17,909	17,849
Securities Sold Under Agreements to Repurchase	120,787	129,577
Payable to Employees and Related Parties	12,964	12,647
Notes Payable	$ 136,860	$ 122,279