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, 2014	Dec. 31, 2013
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 342,908	$ 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
Subordinated Debt Obligations, Fair Value Disclosure	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	7,669	19,134
Accounts Receivable	136,280	83,347
Receivable from Employees and Related Parties	17,327	9,233
Assets Segregated for Bank Regulatory Requirements	0	0
Accounts Payable and Accrued Expenses	37,104	18,365
Securities Sold Under Agreements to Repurchase	106,499	75,563
Payable to Employees and Related Parties	18,875	19,524
Notes Payable	131,340	127,425
Subordinated Debt Obligations, Fair Value Disclosure	22,550
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
Subordinated Debt Obligations, Fair Value Disclosure	0
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	342,908	284,199
Securities Purchased Under Agreements to Resell	7,669	19,134
Accounts Receivable	136,280	83,347
Receivable from Employees and Related Parties	17,327	9,233
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	37,104	18,365
Securities Sold Under Agreements to Repurchase	106,499	75,563
Payable to Employees and Related Parties	18,875	19,524
Notes Payable	131,340	127,425
Subordinated Debt Obligations, Fair Value Disclosure	22,550
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	342,908	284,199
Securities Purchased Under Agreements to Resell	7,669	19,134
Accounts Receivable	136,280	83,347
Receivable from Employees and Related Parties	17,327	9,233
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	37,104	18,365
Securities Sold Under Agreements to Repurchase	106,499	75,563
Payable to Employees and Related Parties	18,875	19,524
Notes Payable	105,226	103,226
Subordinated Debt Obligations, Fair Value Disclosure	$ 22,550