Fair Value Measurements (Fair Value, By Balance Sheet Grouping) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 144,547	$ 136,614
Securities Purchased Under Agreements to Resell	973	2,146
Accounts Receivable	72,666	52,060
Receivable from Employees and Related Parties	5,539	7,793
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	16,278	17,849
Securities Sold Under Agreements to Repurchase	140,443	129,577
Payable to Employees and Related Parties	10,259	12,647
Notes Payable	135,184	122,279
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	144,547	136,614
Securities Purchased Under Agreements to Resell	973	2,146
Accounts Receivable	72,666	52,060
Receivable from Employees and Related Parties	5,539	7,793
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	16,278	17,849
Securities Sold Under Agreements to Repurchase	140,443	129,577
Payable to Employees and Related Parties	10,259	12,647
Notes Payable	100,933	99,664
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	144,547	136,614
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Purchased Under Agreements to Resell	973	2,146
Accounts Receivable	72,666	52,060
Receivable from Employees and Related Parties	5,539	7,793
Accounts Payable and Accrued Expenses	16,278	17,849
Securities Sold Under Agreements to Repurchase	140,443	129,577
Payable to Employees and Related Parties	10,259	12,647
Notes Payable	$ 135,184	$ 122,279