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	Dec. 31, 2013	Dec. 31, 2012
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 284,199	$ 234,568
Securities Purchased Under Agreements to Resell	19,134
Accounts Receivable	83,347	89,098
Receivable from Employees and Related Parties	9,233	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	18,365	17,909
Securities Sold Under Agreements to Repurchase	75,563	120,787
Payable to Employees and Related Parties	19,524	12,964
Notes Payable	103,226	101,375
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	284,199	234,568
Securities Purchased Under Agreements to Resell	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	19,134
Accounts Receivable	83,347	89,098
Receivable from Employees and Related Parties	9,233	5,166
Assets Segregated for Bank Regulatory Requirements	0	0
Accounts Payable and Accrued Expenses	18,365	17,909
Securities Sold Under Agreements to Repurchase	75,563	120,787
Payable to Employees and Related Parties	19,524	12,964
Notes Payable	127,425	136,860
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
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	284,199	234,568
Securities Purchased Under Agreements to Resell	19,134
Accounts Receivable	83,347	89,098
Receivable from Employees and Related Parties	9,233	5,166
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	18,365	17,909
Securities Sold Under Agreements to Repurchase	75,563	120,787
Payable to Employees and Related Parties	19,524	12,964
Notes Payable	$ 127,425	$ 136,860