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	Sep. 30, 2015	Dec. 31, 2014
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 267,842	$ 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
Closely-held Equity Security	0
Loans Receivable	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 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	351	7,669
Accounts Receivable	162,691	136,280
Receivable from Employees and Related Parties	18,814	17,327
Assets Segregated for Bank Regulatory Requirements	0	0
Closely-held Equity Security	0
Loans Receivable	3,653
Accounts Payable and Accrued Expenses	37,809	37,104
Securities Sold Under Agreements to Repurchase	52,025	106,499
Payable to Employees and Related Parties	29,041	18,875
Notes Payable	127,418	131,340
Subordinated Borrowings	23,421	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
Closely-held Equity Security	1,079
Loans Receivable	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 Borrowings	0	0
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	267,842	342,908
Securities Purchased Under Agreements to Resell	351	7,669
Accounts Receivable	162,691	136,280
Receivable from Employees and Related Parties	18,814	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079
Loans Receivable	3,653
Accounts Payable and Accrued Expenses	37,809	37,104
Securities Sold Under Agreements to Repurchase	52,025	106,499
Payable to Employees and Related Parties	29,041	18,875
Notes Payable	127,418	131,340
Subordinated Borrowings	23,421	22,550
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	267,842	342,908
Securities Purchased Under Agreements to Resell	351	7,669
Accounts Receivable	162,691	136,280
Receivable from Employees and Related Parties	18,814	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079
Loans Receivable	3,500
Accounts Payable and Accrued Expenses	37,809	37,104
Securities Sold Under Agreements to Repurchase	52,025	106,499
Payable to Employees and Related Parties	29,041	18,875
Notes Payable	106,829	105,226
Subordinated Borrowings	$ 22,550	$ 22,550