Fair Value Measurements 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, 2014	Dec. 31, 2013
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 221,780	$ 284,199
Securities Purchased Under Agreements to Resell	5,115	19,134
Accounts Receivable	103,397	83,347
Receivable from Employees and Related Parties	12,207	9,233
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	30,291	18,365
Securities Sold Under Agreements to Repurchase	99,484	75,563
Payable to Employees and Related Parties	14,193	19,524
Notes Payable	104,755	103,226
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	221,780	284,199
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Level II [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0	0
Securities Purchased Under Agreements to Resell	5,115	19,134
Accounts Receivable	103,397	83,347
Receivable from Employees and Related Parties	12,207	9,233
Assets Segregated for Bank Regulatory Requirements	0	0
Accounts Payable and Accrued Expenses	30,291	18,365
Securities Sold Under Agreements to Repurchase	99,484	75,563
Payable to Employees and Related Parties	14,193	19,524
Notes Payable	129,702	127,425
Level III [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0	0
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Assets Segregated for Bank Regulatory Requirements	0	0
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	221,780	284,199
Securities Purchased Under Agreements to Resell	5,115	19,134
Accounts Receivable	103,397	83,347
Receivable from Employees and Related Parties	12,207	9,233
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	30,291	18,365
Securities Sold Under Agreements to Repurchase	99,484	75,563
Payable to Employees and Related Parties	14,193	19,524
Notes Payable	$ 129,702	$ 127,425