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	Mar. 31, 2015	Dec. 31, 2014
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 216,281	$ 342,908
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
Short-Term Borrowings	0
Notes Payable	0	0
Subordinated Borrowings	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	5,675	7,669
Accounts Receivable	126,992	136,280
Receivable from Employees and Related Parties	18,547	17,327
Assets Segregated for Bank Regulatory Requirements	0	0
Accounts Payable and Accrued Expenses	36,681	37,104
Securities Sold Under Agreements to Repurchase	94,758	106,499
Payable to Employees and Related Parties	28,516	18,875
Short-Term Borrowings	45,000
Notes Payable	131,390	131,340
Subordinated Borrowings	22,550	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
Short-Term Borrowings	0
Notes Payable	0	0
Subordinated Borrowings	0	0
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	216,281	342,908
Securities Purchased Under Agreements to Resell	5,675	7,669
Accounts Receivable	126,992	136,280
Receivable from Employees and Related Parties	18,547	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	36,681	37,104
Securities Sold Under Agreements to Repurchase	94,758	106,499
Payable to Employees and Related Parties	28,516	18,875
Short-Term Borrowings	45,000
Notes Payable	131,390	131,340
Subordinated Borrowings	22,550	22,550
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	216,281	342,908
Securities Purchased Under Agreements to Resell	5,675	7,669
Accounts Receivable	126,992	136,280
Receivable from Employees and Related Parties	18,547	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Accounts Payable and Accrued Expenses	36,681	37,104
Securities Sold Under Agreements to Repurchase	94,758	106,499
Payable to Employees and Related Parties	28,516	18,875
Short-Term Borrowings	45,000
Notes Payable	105,749	105,226
Subordinated Borrowings	$ 22,550	$ 22,550